CodeGen: Unify two implementations of canDevirtualizeMemberFunctionCall.

They were mostly copy&paste of each other, move it to CodeGenFunction. Of course
the two implementations have diverged over time; the one in CGExprCXX seems to
be the more modern one so I picked that one and moved it to CGClass which feels
like a better home for it. No intended functionality change.

llvm-svn: 189203