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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> @jhuber6 This seems to break the runtimes build. With `CC=clang CXX=clang++ cmake -G Ninja -DCMAKE_EXPORT_COMPILE_COMMANDS=On -DCMAKE_BUILD_TYPE=Debug -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" ../runtimes/` I get
> 
> ```
> CMake Error at CMakeLists.txt:358 (add_custom_command):
>   Attempt to add a custom rule to output
> 
>     <...>/build/compile_commands.json.rule
> 
>   which already has a custom rule.
> ```

Ah, so this seems to be from invoking the runtimes build directly? In that case we probably don't want this to trigger. I wonder if there's a way to detect that. Feel free to revert for now.

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