Add support to the hashing infrastructure for automatically hashing both

Add support to the hashing infrastructure for automatically hashing both
integral and enumeration types. This is accomplished with a bit of
template type trait magic. Thanks to Richard Smith for the core idea
here to detect viable types by detecting the set of types which can be
default constructed in a template parameter.

This is used (in conjunction with a system for detecting nullptr_t
should it exist) to provide an is_integral_or_enum type trait that
doesn't need a whitelist or direct compiler support.

With this, the hashing is extended to the more general facility. This
will be used in a subsequent commit to hashing more things, but I wanted
to make sure the type trait magic went through the build bots separately
in case other compilers don't like this formulation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152217 91177308-0d34-0410-b5e6-96231b3b80d8