[ThinLTO/gold] Perform index-based weak/linkonce resolution

Summary:
Invoke the weak/linkonce symbol resolution support (already used by
libLTO) that operates via the summary index.

This ensures prevailing linkonce are kept, by making them weak, and
marks preempted copies as available_externally when possible.

With this change, the older support for keeping the prevailing linkonce
(by changing their symbol resolution) is removed.

Reviewers: mehdi_amini

Subscribers: llvm-commits, mehdi_amini

Differential Revision: http://reviews.llvm.org/D22302

llvm-svn: 275474