[clang] [flang] [llvm] [mlir] [polly] [CMake][LLVM] Add PCH infrastructure and LLVMSupport PCH (PR #176420)

[Mikael Holmen via cfe-commits]

mikaelholmen wrote:

I see the following when compiling this patch with clang 18.1.8:
```
[...] /repo/llvm/mlir/lib/ExecutionEngine/SparseTensor/Storage.cpp
In file included from <built-in>:1:
In file included from /repo/llvm/llvm/build-all/lib/Support/CMakeFiles/LLVMSupport.dir/cmake_pch.hxx:5:
In file included from /repo/llvm/llvm/include/llvm/Support/pch.h:34:
/repo/llvm/llvm/include/llvm/Support/JSON.h:913:7: error: 'ParseError' has no out-of-line virtual method definitions; its vtable will be emitted in every translation unit [-Werror,-Wweak-vtables]
  913 | class ParseError : public llvm::ErrorInfo<ParseError> {
      |       ^
1 error generated.
```


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