Generate a clang CompilationDatabase when running CMake

This generates a compile_commands.json file, which tells tools like
YouCompleteMe and clang_complete exactly how to build each source file.

Patch by Justin Lebar!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255789 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -86,6 +86,10 @@ set(CMAKE_MODULE_PATH
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
   )
 
+# Generate a CompilationDatabase (compile_commands.json file) for our build,
+# for use by clang_complete, YouCompleteMe, etc.
+set(CMAKE_EXPORT_COMPILE_COMMANDS 1)
+
 option(LLVM_INSTALL_UTILS "Include utility binaries in the 'install' target." OFF)
 
 option(LLVM_INSTALL_TOOLCHAIN_ONLY "Only include toolchain files in the 'install' target." OFF)