[clang][analyzer][NFC] Replace dyn_cast with cast in MemRegion::getMemorySpace

MemRegion::getMemorySpace() is annotated with
LLVM_ATTRIBUTE_RETURNS_NONNULL (which triggers instant UB if a null
pointer is returned), and callers indeed don't check the return value
for null. Thus, even though llvm::dyn_cast is called, it can never
return null in this context. Therefore, we can safely call llvm::cast.

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D151727