Initial support for recursing into the new clang-tools-extra repository

if checked out under clang/tools/extra.

This is mostly so folks other than me can start to test. Documentation,
details, and an announcement are still in the works.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161405 91177308-0d34-0410-b5e6-96231b3b80d8

tools/CMakeLists.txt

@@ -5,3 +5,12 @@ add_subdirectory(c-arcmt-test)
 add_subdirectory(diagtool)
 add_subdirectory(driver)
 add_subdirectory(clang-check)
+
+# We support checking out the clang-tools-extra repository into the 'extra'
+# subdirectory. It contains tools developed as part of the Clang/LLVM project
+# on top of the Clang tooling platform. We keep them in a separate repository
+# to keep the primary Clang repository small and focused.
+if(IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/extra AND
+   EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/extra/CMakeLists.txt)
+  add_subdirectory(extra)
+endif()