[CodeGen] Handle extern references to OBJC_CLASS_$_*

Some ObjC users declare a extern variable named OBJC_CLASS_$_Foo, then use it's
address as a Class. I.e., one could define isInstanceOfF:

BOOL isInstanceOfF(id c) {
  extern void OBJC_CLASS_$_F;
  return [c class] == (Class)&OBJC_CLASS_$_F;
}

This leads to asserts in clang CodeGen if there is an @implementation of F in
the same TU as an instance of this pattern, because CodeGen assumes that a
variable named OBJC_CLASS_$_* has the right type. This commit fixes the problem
by RAUWing the old (incorrectly typed) global with a new global, then removing
the old global.

rdar://45077269

Differential revision: https://reviews.llvm.org/D53154

llvm-svn: 344373