Experiment with using first-class aggregates to represent member function

Experiment with using first-class aggregates to represent member function
pointers.  I find the resulting code to be substantially cleaner, and it
makes it very easy to use the same APIs for data member pointers (which I have
conscientiously avoided here), and it avoids a plethora of potential
inefficiencies due to excessive memory copying, but we'll have to see if it
actually works.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@111776 91177308-0d34-0410-b5e6-96231b3b80d8