Fix hash_integer_value

Broken in D18938 because underlying_type only works for enums and not all stdlibs are sad when given a non-enum. Bots error out with 'only enumeration types have underlying types'.

There's probably a clever enable_if-ism that I can do with underlying_type and the actual integer value, but is_integral_or_enum also accepts implicit conversion so I need to ponder my life choices a bit before committing to template magic. A quick fix for now.

llvm-svn: 265880