LTOCodeGenerator: turns linkonce(_odr) into weak_(odr) when present "MustPreserve" set

Summary:
If the linker requested to preserve a linkonce function, we should
honor this even if we drop all uses.

Reviewers: dexonsmith

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D19527

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 267644