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

[Joseph Huber via llvm-commits]

jhuber6 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.

Yes, if only the projects configure stage runes. If the runtimes configure stage runs afterwards it will trigger that to be updated which will then make the file stale and trigger the merge again. 

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