[Sema] Check whether `__auto_type` has been deduced before merging

This fixes a bug in clang where it emits the following diagnostic when
compiling the test case:

"argument to 'sizeof' in 'memset' call is the same pointer type 'S' as
the destination"

The code that merges __auto_type with other types was committed in
https://reviews.llvm.org/D122029.

Differential Revision: https://reviews.llvm.org/D128373