[analyzer] Add CTU user docs

[dkrupp]

Thanks Gabor for writing this.
I suggested some minor changes to the txt. Otherwise LGTM.

clang/docs/analyzer/user-docs/CrossTranslationUnit.rst

99

since CodeChecker automation is already available I suggest rephrasing.

This manual procedure is error-prone and not scalable, so for analyzing real projects it is recommended to use the built-in support of CodeChecker or scan-build-py.

143

I suggest to replace this line with the following:
E.g. one may use CodeChecker parse to view the results in command line or CodeChecker parse -e html to export them into HTML format.

pure command line usage without worrying about the server setup better fits this command line use-case.

148

Some basic usage description could be added still...

[martong]

• Address dkrupp's comments

[martong]

• Add notes about macro expansion crash with CTU

clang/docs/analyzer/user-docs.rst

 User Docs
 =========
 
 Contents:
+
+.. toctree::
+   :maxdepth: 2
+
+   user-docs/CrossTranslationUnit

clang/docs/analyzer/user-docs/CrossTranslationUnit.rst

+=====================================
+Cross Translation Unit (CTU) Analysis
+=====================================
+
+Normally, static analysis works in the boundary of one translation unit (TU).
+However, with additional steps and configuration we can enable the analysis to inline the definition of a function from another TU.
+
+.. contents::
+   :local:
+
+Manual CTU Analysis
+-------------------
+
+Let's consider these source files in our minimal example:
+
+.. code-block:: cpp
+
+  // main.cpp
+  int foo();
+
+  int main() {
+    return 3 / foo();
+  }
+
+.. code-block:: cpp
+
+  // foo.cpp
+  int foo() {
+    return 0;
+  }
+
+And a compilation database:
+
+.. code-block:: bash
+
+  [
+    {
+      "directory": "/path/to/your/project",
+      "command": "clang++ -c foo.cpp -o foo.o",
+      "file": "foo.cpp"
+    },
+    {
+      "directory": "/path/to/your/project",
+      "command": "clang++ -c main.cpp -o main.o",
+      "file": "main.cpp"
+    }
+  ]
+
+We'd like to analyze `main.cpp` and discover the division by zero bug.
+In order to be able to inline the definition of `foo` from `foo.cpp` first we have to generate the `AST` (or `PCH`) file of `foo.cpp`:
+
+.. code-block:: bash
+
+  $ pwd $ /path/to/your/project
+  $ clang++ -emit-ast -o foo.cpp.ast foo.cpp
+  $ # Check that the .ast file is generated:
+  $ ls
+  compile_commands.json  foo.cpp.ast  foo.cpp  main.cpp
+  $
+
+The next step is to create a CTU index file which holds the `USR` name and location of external definitions in the source files:
+
+.. code-block:: bash
+
+  $ clang-extdef-mapping -p . foo.cpp
+  c:@F@foo# /path/to/your/project/foo.cpp
+  $ clang-extdef-mapping -p . foo.cpp > externalDefMap.txt
+
+We have to modify `externalDefMap.txt` to contain the name of the `.ast` files instead of the source files:
+
+.. code-block:: bash
+
+  $ sed -i -e "s/.cpp/.cpp.ast/g" externalDefMap.txt
+
+We still have to further modify the `externalDefMap.txt` file to contain relative paths:
+
+.. code-block:: bash
+
+  $ sed -i -e "s|$(pwd)/||g" externalDefMap.txt
+
+Now everything is available for the CTU analysis.
+We have to feed Clang with CTU specific extra arguments:
+
+.. code-block:: bash
+
+  $ pwd
+  /path/to/your/project
+  $ clang++ --analyze -Xclang -analyzer-config -Xclang experimental-enable-naive-ctu-analysis=true -Xclang -analyzer-config -Xclang ctu-dir=. -Xclang -analyzer-output=plist-multi-file main.cpp
+  main.cpp:5:12: warning: Division by zero
+    return 3 / foo();
+           ~~^~~~~~~
+  1 warning generated.
+  $ # The plist file with the result is generated.
+  $ ls
+  compile_commands.json  externalDefMap.txt  foo.ast  foo.cpp  foo.cpp.ast  main.cpp  main.plist
+  $
+
+This manual procedure is error-prone and not scalable, therefore to analyze real projects it is recommended to use `CodeChecker` or `scan-build-py`.
+
+Automated CTU Analysis with CodeChecker
+---------------------------------------
+The `CodeChecker <https://github.com/Ericsson/codechecker>`_ project fully supports automated CTU analysis with Clang.
+Once we have set up the `PATH` environment variable and we activated the python `venv` then it is all it takes:
+
+.. code-block:: bash
+
+  $ CodeChecker analyze --ctu compile_commands.json -o reports
+  [INFO 2019-07-16 17:21] - Pre-analysis started.
+  [INFO 2019-07-16 17:21] - Collecting data for ctu analysis.
+  [INFO 2019-07-16 17:21] - [1/2] foo.cpp
+  [INFO 2019-07-16 17:21] - [2/2] main.cpp
+  [INFO 2019-07-16 17:21] - Pre-analysis finished.
+  [INFO 2019-07-16 17:21] - Starting static analysis ...
+  [INFO 2019-07-16 17:21] - [1/2] clangsa analyzed foo.cpp successfully.
+  [INFO 2019-07-16 17:21] - [2/2] clangsa analyzed main.cpp successfully.
+  [INFO 2019-07-16 17:21] - ----==== Summary ====----
+  [INFO 2019-07-16 17:21] - Successfully analyzed
+  [INFO 2019-07-16 17:21] -   clangsa: 2
+  [INFO 2019-07-16 17:21] - Total analyzed compilation commands: 2
+  [INFO 2019-07-16 17:21] - ----=================----
+  [INFO 2019-07-16 17:21] - Analysis finished.
+  [INFO 2019-07-16 17:21] - To view results in the terminal use the "CodeChecker parse" command.
+  [INFO 2019-07-16 17:21] - To store results use the "CodeChecker store" command.
+  [INFO 2019-07-16 17:21] - See --help and the user guide for further options about parsing and storing the reports.
+  [INFO 2019-07-16 17:21] - ----=================----
+  [INFO 2019-07-16 17:21] - Analysis length: 0.659618854523 sec.
+  $ ls
+  compile_commands.json  foo.cpp  foo.cpp.ast  main.cpp  reports
+  $ tree reports
+  reports
+  ├── compile_cmd.json
+  ├── compiler_info.json
+  ├── foo.cpp_53f6fbf7ab7ec9931301524b551959e2.plist
+  ├── main.cpp_23db3d8df52ff0812e6e5a03071c8337.plist
+  ├── metadata.json
+  └── unique_compile_commands.json
+
+  0 directories, 6 files
+  $
+
+The `plist` files contain the results of the analysis, which may be viewed with the regular analysis tools.
+E.g. one may use `CodeChecker parse` to view the results in command line:
+
+.. code-block:: bash
+
+  $ CodeChecker parse reports
+  [HIGH] /home/egbomrt/ctu_mini_raw_project/main.cpp:5:12: Division by zero [core.DivideZero]
+    return 3 / foo();
+             ^
+
+  Found 1 defect(s) in main.cpp
+
+
+  ----==== Summary ====----
+  -----------------------
+  Filename | Report count
+  -----------------------
+  main.cpp |            1
+  -----------------------
+  -----------------------
+  Severity | Report count
+  -----------------------
+  HIGH     |            1
+  -----------------------
+  ----=================----
+  Total number of reports: 1
+  ----=================----
+
+Or we can use `CodeChecker parse -e html` to export the results into HTML format:
+
+.. code-block:: bash
+
+  $ CodeChecker parse -e html -o html_out reports
+  $ firefox html_out/index.html
+
+If you experience that Clang crashes during the visitation of the BugPath then please disable macro expansion when CTU is used:
+
+.. code-block:: bash
+
+  $ echo "-Xclang -analyzer-stats -Xclang -analyzer-config -Xclang expand-macros=false" >./saargs_file
+  $ CodeChecker analyze --ctu --saargs ./saargs_file compile_commands.json -o reports
+
+We have a patch which will solve this issue soon.
+
+Automated CTU Analysis with scan-build-py (don't do it)
+-------------------------------------------------------
+We actively develop CTU with CodeChecker as a "runner" script, `scan-build-py` is not actively developed for CTU.
+`scan-build-py` has various errors and issues, expect it to work with the very basic projects only.
+
+Example usage of scan-build-py:
+
+.. code-block:: bash
+
+  $ /your/path/to/llvm-project/clang/tools/scan-build-py/bin/analyze-build --ctu
+  analyze-build: Run 'scan-view /tmp/scan-build-2019-07-17-17-53-33-810365-7fqgWk' to examine bug reports.
+  $ /your/path/to/llvm-project/clang/tools/scan-view/bin/scan-view /tmp/scan-build-2019-07-17-17-53-33-810365-7fqgWk
+  Starting scan-view at: http://127.0.0.1:8181
+    Use Ctrl-C to exit.
+  [6336:6431:0717/175357.633914:ERROR:browser_process_sub_thread.cc(209)] Waited 5 ms for network service
+  Opening in existing browser session.
+  ^C
+  $