Implement LWG 2148: Make non-enum default hash specialization well-formed

Implement LWG 2148: Make non-enum default hash specialization well-formed

Summary:
This patch removes the static_assert for non-enum types in the primary hash template. Instead non-enum types create a hash<T> specialization that is not constructible nor callable.

See also:
  * http://cplusplus.github.io/LWG/lwg-active.html#2543
  * https://llvm.org/bugs/show_bug.cgi?id=28917

Reviewers: mclow.lists, EricWF

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D23331

llvm-svn: 278300