[clang][Interp] Check This pointer without creating InterpFrame

The InterpFrame was only created so early so we could use getThis().
However, we need to know the Function when creating the InterpFrame and
in the case of virtual functions, we need to determine what function to
call at interpretation time.

Get the This pointer ourselves and just create the InterpFrame later.

Differential Revision: https://reviews.llvm.org/D142617