Prevent Scoped Enums from being Integral constant expressions:

Discovered because of: https://bugs.llvm.org/show_bug.cgi?id=38235

It seems to me that a scoped enum should NOT be an integral constant expression
without a cast, so this seems like a sensical change.

Attributes that check for an integer parameter simply use this function to
ensure that they have an integer, so it was previously allowing a scoped enum.

Also added a test based on Richard's feedback to ensure that case labels still work.

Differential Revision: https://reviews.llvm.org/D49599

llvm-svn: 337585