[ThinLTO] Remove functions resolved to available_externally from comdats

Summary:
thinLTOResolveWeakForLinkerModule needs to drop any preempted weak symbols
that were converted to available_externally from comdats, otherwise we
will get a verification failure (since available_externally is a
declaration for the linker, and no declarations can be in a comdat).

Reviewers: mehdi_amini

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23015

llvm-svn: 278739