Make all virtual member pointers use variadic musttail calls

This avoids encoding information about the function prototype into the
thunk at the cost of some function prototype bitcast gymnastics.

Fixes PR20653.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216782 91177308-0d34-0410-b5e6-96231b3b80d8