LTO: Don't bother trying to mangle unnamed globals, as they can't be preserved...

LTO: Don't bother trying to mangle unnamed globals, as they can't be preserved with MustPreserveSymbols.

Summary: Should fix sanitizer-windows bot.

Reviewers: joker.eph

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D19635

llvm-svn: 267820