[clang] [clang] fix clang_cmake_builddir (PR #155844)

[Peter Waller via cfe-commits]

peterwaller-arm wrote:

Hi Folks, this results in `.cmake` files being written to the installation directory of LLVM. Is that intentional? If this is a system installation, this is not desirable, since it may be read-only, resulting in error messages of this form.

```
CMake Error: Could not open file for write in copy operation /nix/store/x63p9dancrmkihdqqvjri1pmmjab2hl4-llvm-22.0.0-git-2025-09-15-lib/lib/cmake/clang/ClangConfig.cmake.tmp
CMake Error: : System Error: No such file or directory
CMake Error at cmake/modules/CMakeLists.txt:31 (configure_file):
  configure_file Problem configuring file
>
CMake Error: Could not open file for write in copy operation /nix/store/x63p9dancrmkihdqqvjri1pmmjab2hl4-llvm-22.0.0-git-2025-09-15-lib/lib/cmake/clang/ClangConfigVersion.cmake.tmp
CMake Error: : System Error: No such file or directory
CMake Error at cmake/modules/CMakeLists.txt:35 (configure_file):
  configure_file Problem configuring file
```

This affects the nixpkgs build (likely others), and anywhere llvm is installed separately from clang: https://github.com/NixOS/nixpkgs/pull/443041

Please could we entertain a revert and alternative solution?

https://github.com/llvm/llvm-project/pull/155844