[lto] Don't add variables with private linkage to the symbol table.

Summary:
This causes the issue in PR26872 to go away now that we aren't creating
symbols for the string literals, but that may just be concealing a
deeper problem, so best to keep that PR open.

Reviewers: rafael

Subscribers: Bigcheese, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D17971

llvm-svn: 262968