[analyzer] Explicit cast on customized offsetof should not be ignored when evaluating as const

If ignored, the subexpr is a UnaryOperator (&) which cannot be evaluated
(assertion failed).

  #define offsetof(type,memb) ((unsigned long)&((type*)0)->memb)

Patch By danix800!

Differential Revision: https://reviews.llvm.org/D144780

(cherry picked from commit 53f75425)