[analyzer] Fix assertion in SVals.h

Fix assertion in SVals.h apparently caused by
https://reviews.llvm.org/D89055.

clang:clang/include/clang/StaticAnalyzer/Core/PathSensitive/SVals.h:596:
clang::ento::loc::MemRegionVal::MemRegionVal(const clang::ento::MemRegion *):
  Assertion `r' failed.

Backtrace:
...
     clang/include/clang/StaticAnalyzer/Core/PathSensitive/SVals.h:597:3
     clang::QualType, clang::QualType)
     clang/lib/StaticAnalyzer/Core/SValBuilder.cpp:773:18
     clang::QualType, clang::QualType)
     clang/lib/StaticAnalyzer/Core/SValBuilder.cpp:612:12
     clang::QualType) clang/lib/StaticAnalyzer/Core/SValBuilder.cpp:587:12
     namespace)::RegionBindingsRef const&, clang::ento::Loc, clang::QualType)
     clang/lib/StaticAnalyzer/Core/RegionStore.cpp:1510:24
...

Reviewed By: ASDenysPetrov

Differential Revision: https://reviews.llvm.org/D101635