aengelke wrote:

> I think we should give a warning message if sccache + pch is activated.

Added warnings for typical CMAKE_CXX_COMPILER_LAUNCHER values.

> how do we enable PCH? Is there some standard cmake flag or an llvm-specific 
> one? Could you include this in the PR description?

Added a note that PCH is on-by-default unless noted as exception.

> Err, how is it going to interact with downstream projects that don't disable 
> PCH for C files?

If the downstream just uses standard CMake add_library etc., nothing changes. 
If the downstream uses AddLLVM (e.g., add_llvm_library) for a C-only target and 
doesn't specify `DISABLE_PCH_REUSE`, then CMake will fail with an error similar 
to:
```
CMake Error in examples/OrcV2Examples/OrcV2CBindingsBasicUsage/CMakeLists.txt:
  Unable to resolve full path of PCH-header
  '/path/to/llvm/build/lib/Support/CMakeFiles/LLVMSupport.dir/cmake_pch.h'
  assigned to target OrcV2CBindingsBasicUsage, although its path is supposed
  to be known!
```


https://github.com/llvm/llvm-project/pull/176420
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