A few tweaks to the value-kind computation:

 - Default argument expressions pick up the value kind of the incoming
   expression, not the value kind of the parameter it initializes.
 - When building a template argument for substitution, A::x is an rvalue
   if x is an instance method.
 - Anonymous struct/union paths pick up value kind the same way that
   normal member accesses do;  extract out a common code path for this.

Enable the value-kind assertion, now that it passes self-host.

llvm-svn: 120055