Use musttail for variadic method thunks when possible

This avoids cloning variadic virtual methods when the target supports
musttail and the return type is not covariant. I think we never
implemented this previously because it doesn't handle the covariant
case. But, in the MS ABI, there are some cases where vtable thunks must
be emitted even when the variadic method defintion is not available, so
it looks like we need to implement this. Do it for both ABIs, since it's
a nice size improvement and simplification for Itanium.

Emit an error when emitting thunks for variadic methods with a covariant
return type. This case is essentially not implementable unless the ABI
provides a way to perfectly forward variadic arguments without a tail
call.

Fixes PR43173.

Differential Revision: https://reviews.llvm.org/D67028

llvm-svn: 371269