We want lld-link to automatically find compiler-rt's and
libc++ when it's in the same directory as the rest of the
toolchain. This is because on Windows linking isn't done
via the clang driver - but instead invoked directly.
This prepends: <llvm>/lib <llvm>/lib/clang/XX/lib and
<llvm>/lib/clang/XX/lib/windows automatically to the library
search paths.
Related to #63827
Differential Revision: https://reviews.llvm.org/D151188
af744f0b84