[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;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164441 91177308-0d34-0410-b5e6-96231b3b80d8