Add new LLVM_EXTERNAL_PROJECTS option to cmake

[tberghammer]

Summary

Add new LLVM_EXTERNAL_PROJECTS option to cmake

The new option makes it possible to build external projects as part of
the llvm build without copying (or symlinking) then into llvm/tool with
specifying a few additional cmake variables.

Example usage (2 additional project called foo and bar):
-DLLVM_EXTERNAL_PROJECTS="Foo;Bar"
-DLLVM_EXTERNAL_FOO_SOURCE_DIR=/src/foo
-DLLVM_EXTERNAL_BAR_SOURCE_DIR=/src/bar

Note: This is the extension of the approach we already support for
clang/lldb/poly with adding an option to specify additional supported
projects.

[beanz]

LGTM.

docs/CMake.rst

 **LLVM_EXTERNAL_{CLANG,LLD,POLLY}_SOURCE_DIR**:PATH
   These variables specify the path to the source directory for the external
   LLVM projects Clang, lld, and Polly, respectively, relative to the top-level
   source directory.  If the in-tree subdirectory for an external project
   exists (e.g., llvm/tools/clang for Clang), then the corresponding variable
   will not be used.  If the variable for an external project does not point
   to a valid path, then that project will not be built.
 
+**LLVM_EXTERNAL_PROJECTS**:STRING
+  Semicolon-separated list of additional external projects to build as part of
+  llvm. For each project LLVM_EXTERNAL_<NAME>_SOURCE_DIR have to be specified
+  with the path for the source code of the project. Example:
+  ``-DLLVM_EXTERNAL_PROJECTS="Foo;Bar"
+    -DLLVM_EXTERNAL_FOO_SOURCE_DIR=/src/foo
+    -DLLVM_EXTERNAL_BAR_SOURCE_DIR=/src/bar``.
+
 **LLVM_USE_OPROFILE**:BOOL
   Enable building OProfile JIT support. Defaults to OFF.
 
 **LLVM_PROFDATA_FILE**:PATH
   Path to a profdata file to pass into clang's -fprofile-instr-use flag. This
   can only be specified if you're building with clang.
 
 **LLVM_USE_INTEL_JITEVENTS**:BOOL

tools/CMakeLists.txt

 add_llvm_external_project(llgo)
 add_llvm_external_project(lld)
 add_llvm_external_project(lldb)
 
 # Automatically add remaining sub-directories containing a 'CMakeLists.txt'
 # file as external projects.
 add_llvm_implicit_projects()
 
+# Add subprojects specified using LLVM_EXTERNAL_PROJECTS
+foreach(p ${LLVM_EXTERNAL_PROJECTS})
+  add_llvm_external_project(${p})
+endforeach(p)
+
 set(LLVM_COMMON_DEPENDS ${LLVM_COMMON_DEPENDS} PARENT_SCOPE)