is_integral_or_enum ❥ enum class ⇒ hashable enum class

Summary:
As discussed in D18775 making AtomicOrdering an enum class makes it non-hashable, which shouldn't be the case. Hashing.h defines hash_value for all is_integral_or_enum, but type_traits.h's definition of is_integral_or_enum only checks for *inplicit* conversion to integral types which leaves enum classes out and is very confusing because is_enum is true for enum classes.

This patch:
  - Adds a check for is_enum when determining is_integral_or_enum.
  - Explicitly converts the value parameter in hash_value to handle enum class hashing.

Note that the warning at the top of Hashing.h still applies: each execution of the program has a high probability of producing a different hash_code for a given input. Thus their values are not stable to save or persist, and should only be used during the execution for the construction of hashing datastructures.

Reviewers: dberlin, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18938

llvm-svn: 265879