[llvm] [Runtimes] Merge 'compile_commands.json' files from runtimes build (PR #116303)

[Petr Hosek via llvm-commits]

petrhosek wrote:

This slightly changes the semantics of `compile_commands.json` generation. Normally, `compile_commands.json` is generated by CMake during the configuration stage which means that you don't need to run build at all; after this change, we'll generate the top-level `compile_commands.json` every time we (re-)run CMake, but that file will be replaced by the merged one during the build.

https://github.com/llvm/llvm-project/pull/116303