Driver: use ld64.lld when -fuse-ld=lld for darwin

When using lld on macOS the current level of detection between ld and
ld64 forces us to rename lld to ld.

For ELF targets we have the ld.lld alias so for MACHO we should have
ld64.lld so we can use lld without replacing the system compiler.

This also solves the additional issue of cross compiling for MACHO
where renaming lld to ld with only target ELF.

This is the clang driver component change to use this new alias.

Reviewers: ruiu, rnk

Differential Revision: https://reviews.llvm.org/D38290

llvm-svn: 315867