[analyzer] Don't look through casts when creating pointer temporaries.
Normally, we need to look through derived-to-base casts when creating
temporary object regions (added in r175854). However, if the temporary
is a pointer (rather than a struct/class instance), we need to /preserve/
the base casts that have been applied.
This also ensures that we really do create a new temporary region when
we need to: MaterializeTemporaryExpr and lvalue CXXDefaultArgExprs.
Fixes PR15342, although the test case doesn't include the crash because
I couldn't isolate it.