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

[Joseph Huber via llvm-commits]

jhuber6 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.

Well you'll still get the normal `compile_commands.json` with a configure, but since the runtimes directories are only triggered during the build it will then merge them in.

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