Format strings: check against an enum's underlying type.

This allows us to be more careful when dealing with enums whose fixed
underlying type requires special handling in a format string, like
NSInteger.

A refinement of r163266 from a year and a half ago, which added the
special handling for NSInteger and friends in the first place.

<rdar://problem/16616623>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209966 91177308-0d34-0410-b5e6-96231b3b80d8