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

[Nathan Gauër via cfe-commits]

Keenuts wrote:

Thanks, I can reproduce with the following steps:

```
$ cmake -Hllvm -GNinja -B/tmp/nix-llvm/ -DBUILD_TESTING=OFF \
  -DCLANGD_BUILD_XPC:BOOL=FALSE -DLLVM_ENABLE_RTTI:BOOL=TRUE \
  -DLLVM_INCLUDE_TESTS:BOOL=FALSE -DCMAKE_BUILD_TYPE=Debug \
  -DLLVM_OPTIMIZED_TABLEGEN=1 -DLLVM_ENABLE_LLD=1 \
  -DCMAKE_LINKER=lld-link -DLLVM_USE_SPLIT_DWARF=1 \
  -DCMAKE_INSTALL_PREFIX=/tmp/nix-llvm-install
$ ninja -C /tmp/nix-llvm install
$ chmod -R -w /tmp/nix-llvm-install
$ cmake -Hclang -GNinja -B/tmp/nix-clang/ -DBUILD_TESTING=OFF \
  -DCLANGD_BUILD_XPC:BOOL=FALSE -DLLVM_ENABLE_RTTI:BOOL=TRUE \
  -DLLVM_INCLUDE_TESTS:BOOL=FALSE -DCMAKE_BUILD_TYPE=Debug \
  -DLLVM_OPTIMIZED_TABLEGEN=1 -DLLVM_ENABLE_LLD=1 \
  -DCMAKE_LINKER=lld-link -DLLVM_USE_SPLIT_DWARF=1 \
  -DCMAKE_CXX_COMPILER=clang++-17 -DCMAKE_C_COMPILER=clang-17 \
  -DLLVM_DIR:STRING=/tmp/nix-llvm-install/lib/cmake/llvm \
  -DLLVM_LINK_LLVM_DYLIB=ON \
  -DCMAKE_INSTALL_PREFIX=/tmp/nix-clang-install
-- Building with -fPIC
-- Clang version: 22.0.0git
CMake Error: Could not open file for write in copy operation /tmp/nix-llvm-install/lib/cmake/clang/ClangConfig.cmake.tmp
CMake Error: : System Error: Permission denied
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 /tmp/nix-llvm-install/lib/cmake/clang/ClangConfigVersion.cmake.tmp
CMake Error: : System Error: Permission denied
CMake Error at cmake/modules/CMakeLists.txt:35 (configure_file):
  configure_file Problem configuring file
```


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