CodeGen: Assign linkage to thread-wrappers correctly

We would sometimes incorrectly give a thread-wrapper external linkage
instead of internal linkage if we had only CodeGen'd it's declaration,
not it's definition.

This fixes PR19655.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207988 91177308-0d34-0410-b5e6-96231b3b80d8