CodeGen: Properly null-check typeid expressions

Thoroughly check for a pointer dereference which yields a glvalue.  Look
through casts, comma operators, conditional operators, paren
expressions, etc.

This was originally D4416.

Differential Revision: http://reviews.llvm.org/D4592

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213434 91177308-0d34-0410-b5e6-96231b3b80d8