When CLANG_RESOURCE_DIR is not empty, clang will look up builtin headers in CLANG_RESOURCE_DIR/include, this patch makes sure resource headers are installed into the correct location.
Resolve issue #57708.
The property we maintain is that the build directory layout matches the installation directory which makes it possible to use the toolchain without having to install it first.
I believe we need to set the output_dir depending on the value of CLANG_RESOURCE_DIR as well to preserve that behavior.
nit: can we use if ... else?