[lld/mac] Stop accepting arbitrary suffixes on --(no-)warn-dylib-install-name

The flag accidentally used Joined<> instead of Flag<>.

Previously, `--warn-dylib-install-namefoobarbaz` would be accepted and
had the same effect as `-warn-dylib-install-name`. Now the flag only
works if no suffix is attached to it, as originally intended.

Also fix a typo in the flag's help text.

Differential Revision: https://reviews.llvm.org/D131781