Inline asm constraints: allow ICE-like pointers for the "n" constraint (PR40890)

Apparently GCC allows this, and there's code relying on it (see bug).

The idea is to allow expression that would have been allowed if they
were cast to int. So I based the code on how such a cast would be done
(the CK_PointerToIntegral case in IntExprEvaluator::VisitCastExpr()).

Differential Revision: https://reviews.llvm.org/D58821

llvm-svn: 355491