[CFI/MergeFunctions] Modify MergeFunctions to propagate type information (#68628)

When MergeFuncs creates a thunk, it does not modify the function in
place, but creates a new one altogether. If type metadata is not
properly forwarded to this new function, LowerTypeTests will be unable
to put this thunk into the dispatch table.

The fix here is to just forward the type metadata to the newly created
functions.