YAMLIO: Improve endian type support

Summary:
Now that endian types support enumerations (D59141), the existing yaml
support for them is somewhat insufficient. The current solution was to
define the ScalarTraits class for these types, which always forwards to
the ScalarTraits of the underlying type. However, the enum types will
usually have ScalarEnumerationTraits of ScalarBitsetTraits.

In this patch I add the two extra Traits types to the endian types. In
order to properly SFINAE-ize them, I've also added an extra "Enable"
template argument to the Traits template classes.

Reviewers: zturner, sammccall

Subscribers: kristina, Bigcheese, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59289

llvm-svn: 356269