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
1 file changed