Generate a compile_commands.json DB for external projects.

compile_commands.json file is very useful both for tooling and for
reproducible builds.
For files generated from recursive CMake invocation this information was
not previously generated.

Differential Review: https://reviews.llvm.org/D35219

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308698 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/LLVMExternalProjectUtils.cmake

@@ -149,6 +149,7 @@ function(llvm_ExternalProject_Add name source_dir)
                -DPACKAGE_VERSION=${PACKAGE_VERSION}
                -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
                -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
+               -DCMAKE_EXPORT_COMPILE_COMMANDS=1
                ${ARG_CMAKE_ARGS}
                ${PASSTHROUGH_VARIABLES}
     INSTALL_COMMAND ""