[tooling] Explain how to create a compilation database on Windows [NFC]

[LegalizeAdulthood]

Summary

Document how to create a compile_commands.json file on Windows

[aaron.ballman]

LGTM aside from a formatting nit, thank you -- this is really helpful information! I confirmed the instructions work for me on Windows.

clang/docs/HowToSetupToolingForLLVM.rst

66–68

Looks like you can re-flow this to the usual 80 col limit.

[LegalizeAdulthood]

Update from review comments

clang/docs/HowToSetupToolingForLLVM.rst

.. code-block:: console
   $ ln -s $PWD/compile_commands.json path/to/llvm/source/
 
 Now you are ready to build and test LLVM using make:
 
 .. code-block:: console
 
   $ make check-all
 
+Setup Clang Tooling Using CMake on Windows
+==========================================
+
+For Windows developers, the Visual Studio project generators in CMake do
+not support `CMAKE_EXPORT_COMPILE_COMMANDS
+<https://cmake.org/cmake/help/latest/variable/CMAKE_EXPORT_COMPILE_COMMANDS.html>`_.
+However, the Ninja generator does support this variable and can be used
+on Windows to generate a suitable ``compile_commands.json`` that invokes
+the MSVC compiler.
+
+First, you will need to install `Ninja`_.  Once installed, the Ninja
+executable will need to be in your search path for CMake to locate it.
+
+Next, assuming you already have Visual Studio installed on your machine, you
+need to have the appropriate environment variables configured so that CMake
+will locate the MSVC compiler for the Ninja generator.  The `documentation
+<https://docs.microsoft.com/en-us/cpp/build/building-on-the-command-line?view=msvc-170#path_and_environment>`_
+describes the necessary environment variable settings, but the simplest thing
+is to use a `developer command-prompt window
+<https://docs.microsoft.com/en-us/cpp/build/building-on-the-command-line?view=msvc-170#developer_command_prompt_shortcuts>`_
+or call a `developer command file
+<https://docs.microsoft.com/en-us/cpp/build/building-on-the-command-line?view=msvc-170#developer_command_file_locations>`_
+to set the environment variables appropriately.
+
+Now you can run CMake with the Ninja generator to export a compilation
+database:
+
+.. code-block:: console
+
+  C:\> mkdir build-ninja
+  C:\> cd build-ninja
+  C:\build-ninja> cmake -G Ninja -DCMAKE_EXPORT_COMPILE_COMMANDS=ON path/to/llvm/sources
+
+It is best to keep your Visual Studio IDE build folder separate from the
+Ninja build folder.  This prevents the two build systems from negatively
+interacting with each other.
+
+Once the ``compile_commands.json`` file has been created by Ninja, you can
+use that compilation database with Clang Tooling.  One caveat is that because
+there are indirect settings obtained through the environment variables,
+you may need to run any Clang Tooling executables through a command prompt
+window created for use with Visual Studio as described above.  An
+alternative, e.g. for using the Visual Studio debugger on a Clang Tooling
+executable, is to ensure that the environment variables are also visible
+to the debugger settings.  This can be done locally in Visual Studio's
+debugger configuration locally or globally by launching the Visual Studio
+IDE from a suitable command-prompt window.
+
 Using Clang Tools
 =================
 
 After you completed the previous steps, you are ready to run clang tools. If
 you have a recent clang installed, you should have ``clang-check`` in
 ``$PATH``. Try to run it on any ``.cpp`` file inside the LLVM source tree:
 
 .. code-block:: console
clang::ASTConsumer *newASTConsumer() {
       if (this->ASTPrint.operator _Bool())
           return clang::CreateASTPrinter(&llvm::outs(), this->ASTDumpFilter);
       return new clang::ASTConsumer();
   }
 
 Using Ninja Build System
 =======================================
 
-Optionally you can use the `Ninja <https://github.com/martine/ninja>`_
-build system instead of make. It is aimed at making your builds faster.
-Currently this step will require building Ninja from sources.
+Optionally you can use the `Ninja`_ build system instead of make. It is
+aimed at making your builds faster.  Currently this step will require
+building Ninja from sources.
 
 To take advantage of using Clang Tools along with Ninja build you need
 at least CMake 2.8.9.
 
 Clone the Ninja git repository and build Ninja from sources:
 
 .. code-block:: console
 

 
 Now you are ready to build and test LLVM using Ninja:
 
 .. code-block:: console
 
   $ ninja check-all
 
 Other target names can be used in the same way as with make.
+
+.. _Ninja: https://ninja-build.org/