PR13051: If a constructor is explicitly defaulted, it isn't marked as being

constexpr until we get to the end of the class definition. When that happens,
be sure to remember that the class actually does have a constexpr constructor.

This is a stopgap solution, which still doesn't cover the case of a class with
multiple copy constructors (only some of which are constexpr). We should be
performing constructor lookup when implicitly defining a constructor in order
to determine whether all constructors it invokes are constexpr.

llvm-svn: 158228