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

[Nathan Gauër via cfe-commits]

Keenuts wrote:

(@rjodinchr FYI)

Hi, so to understand, is it when you do something like `cmake -DCMAKE_INSTALL_DIR=some/privileged/location` then this writes to the INSTALL_DIR when doing `ninja -C build` (so before the installation step?)

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