[Clang] Add option to set alternative toolchain path

In some cases, we need to set alternative toolchain path other than the
default with system (headers, libraries, dynamic linker prefix, ld path,
etc.), e.g., to pick up newer components, but keep sysroot at the same
time (to pick up extra packages).

This change introduces a new option --overlay-platform-toolchain to set
up such alternative toolchain path.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D121992