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

[Shilei Tian via llvm-commits]

shiltian wrote:

Just want to confirm that,, when the top level configure runs again, the top level `compile_commands.json` will be overridden with only the top level project related commands right? Those commands added by the merge will be simply gone. If that is the case, I'm okay with the change. However, if that's not the case, I don't think this change is valid.

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