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
Akira Hatanaka