[lto] Avoid duplicate symbol error for appending linkage.

Summary:
Is there any other code needed for correctly handling appending linkage?
Do we need to do something more with @llvm.global_ctors in
SymbolTable.cpp:addBitcodeFile; otherwise the combined bitcode module
won't have all the global ctors.

Reviewers: rafael

Subscribers: Bigcheese, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D17975

llvm-svn: 262992