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

[Nikolas Klauser via llvm-commits]

philnik777 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.
```

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