Plumb native-clang-tools-path to build support. (#81587)
The build support Python libraries assume by default that if we do not
supply a Swift toolchain path, we can find clang in the installed
toolchain path: i.e., the clang that we just built. However, possibly
during bootstrap, we may not have a preexisting Swift compiler but still
want to use the clang on the platform that is already installed.
build-script already gives us native-clang-tools-path. Here, we plumb
this through to the relevant Python modules. If the
native-clang-tools-path is not specified, we use the
install_toolchain_path, just like native_toolchain_path, and the
existing behavior is effectively unchanged. If we do specify a
native-clang-tools-path, then we return it to ensure that we properly
refer to the clang that lives there instead of always defaulting to the
just-built clang.
