Add Available Externally linkage type to isWeakForLinker()

Per LangRef: "Globals with available_externally linkage are
allowed to be discarded at will, and are otherwise the same
as linkonce_odr", since linkonce_odr is in this list it makes
sense to have available_externally there as well.

Reviewers: rafael

Differential Revision: http://reviews.llvm.org/D15323

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 255043