[ADT] Implement Any::has_value

This patch implements Any::has_value for consistency with std::any in
C++17.

My plan is to deprecate Any::hasValue after migrating all of its uses
to Any::has_value.  Since I am about to do so, this patch simply
replaces hasValue with has_value in the unit test instead of adding
tests for has_value.

Differential Revision: https://reviews.llvm.org/D132278