CodeGen: Don't drop thread_local when emitting __thread aliases

CodeGen wouldn't mark the aliasee as thread_local if the aliasee was a
tentative definition.

Even if the definition was already emitted, it would never mark the
alias as thread_local.

This fixes PR21288.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219859 91177308-0d34-0410-b5e6-96231b3b80d8