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

[Petr Hosek via llvm-commits]

petrhosek wrote:

Have you measured at the performance on larger builds? For example, in the [Fuchsia toolchain build](https://github.com/llvm/llvm-project/blob/d681e1030ffd71412294d3fadc7ef673f822b832/clang/cmake/caches/Fuchsia.cmake) that I normally use, I have 38 `compile_commands.json` files, the top-level one is 12M and the individual runtime ones are around 1-2M.  Python is not the fastest language out there and I just want to make sure this doesn't become a bottleneck even for larger builds.

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