Suppress Wsign-conversion for enums with matching underlying type

As reported here: https://bugs.llvm.org/show_bug.cgi?id=34692

A non-defined enum with a backing type was always defaulting to
being treated as a signed type. IN the case where it IS defined,
the signed-ness of the actual items is used.

This patch uses the underlying type's signed-ness in the non-defined
case to test signed-comparision.

Differential Revision: https://reviews.llvm.org/D38145

llvm-svn: 313907