[analyzer] Check that a member expr is valid even when the result is an lvalue.

[analyzer] Check that a member expr is valid even when the result is an lvalue.

We want to catch cases like this early, so that we can produce better
diagnostics and path notes:

  Point *p = 0;
  int *px = &p->x; // should warn here
  *px = 1;

llvm-svn: 164441