Generating available_externally vtables for outline virtual functions

Generating available_externally vtables for optimizations purposes.
Unfortunatelly ItaniumABI doesn't guarantee that we will be able to
refer to virtual inline method by name.
But when we don't have any inline virtual methods, and key function is
not defined in this TU, we can generate that there will be vtable and
mark it as available_externally.

This is patch will help devirtualize better.
Differential Revision: http://reviews.llvm.org/D11441

llvm-svn: 243090