Do not emit thunks with available_externally linkage in comdats

Functions with available_externally linkage will not be emitted to object
files (they will just be undefined symbols), so it does not make sense to
put them in comdats.

Creates a second overload of maybeSetTrivialComdat that uses the GlobalObject
instead of the Decl, and uses that in several places that had the faulty
logic.

Differential Revision: http://reviews.llvm.org/D9580

llvm-svn: 236879