Make GlobalValues with non-default visibilility dso_local.

This is similar to r322317, but for visibility. It is not as neat
because we have to special case extern_weak.

The idea is the same as the previous change, make the transition to
explicit dso_local easier for the frontends. With this they only have
to add dso_local to symbols where we need some external information to
decide if it is dso_local (like it being part of an ELF executable).

llvm-svn: 322806