[ubsan] Null-check and adjust TypeLoc before using it

Null-check and adjut a TypeLoc before casting it to a FunctionTypeLoc.
This fixes a crash in -fsanitize=nullability-return, and also makes the
location of the nonnull type available when the return type is adjusted.

rdar://59263039

Differential Revision: https://reviews.llvm.org/D74355