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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196658 91177308-0d34-0410-b5e6-96231b3b80d8