Fix self-host; if a thunk already exists and has available_externally linkage,...

Fix self-host; if a thunk already exists and has available_externally linkage, we should change its linkage instead of asserting.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124986 91177308-0d34-0410-b5e6-96231b3b80d8