Normally, passing -rtlib=platform overrides any earlier -rtlib
options, and overrides any hardcoded CLANG_DEFAULT_RTLIB option.
However, some targets, MSVC and Darwin, have custom logic for
disallowing specific -rtlib= option values; amend these checks for
allowing the -rtlib=platform option.
Differential Revision: https://reviews.llvm.org/D132444
Martin Storsjö