[lld/mac] warn on -install_name without -dylib

The flag doesn't (and shouldn't) have an effect in that case.
ld64 doesn't warn on this, but it seems like a good thing to do.
If it causes problems in practice for some reason, we can revert it.

Also add a dedicated test for install_name.

Differential Revision: https://reviews.llvm.org/D98259