[modules] Defer merging deduced return types.

We can't read a deduced return type until we are sure that the types referred
to by it are not in the middle of being loaded. So defer all reading of such
deduced return types until the end of the recursive deserialization step.

Also, when we load a function type that has a deduced return type, update all
other redeclarations of the function to have that deduced return type.

llvm-svn: 338798