Make yet another placeholder type, this one marking that an expression is a bound

member function, i.e. something of the form 'x.f' where 'f' is a non-static
member function.  Diagnose this in the general case.  Some of the new diagnostics
are probably worse than the old ones, but we now get this right much more
universally, and there's certainly room for improvement in the diagnostics.

llvm-svn: 130239