Merging r196658:
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r196658 | d0k | 2013-12-07 08:12:52 -0800 (Sat, 07 Dec 2013) | 7 lines

CodeGen: Don't emit linkage on thunks that aren't emitted because they're vararg.

This can happen when we're trying to emit a thunk with available_externally
linkage with optimization enabled but bail because it doesn't make sense
for vararg functions.

PR18098.
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_34@196666 91177308-0d34-0410-b5e6-96231b3b80d8