[clang] Make --ld-path= work with -fuse-ld=lld

This allows using --ld-path= to set a custom linker path, while
still informing clang that the binary at that path is an lld built
at the same revision as clang, so that clang can make assumptions
about the flags it supports, its output format, etc.

This currently only has an observable effect on Darwin.

Differential Revision: https://reviews.llvm.org/D134063