Improve key-function computation for templates. In particular:

Improve key-function computation for templates. In particular:
  - All classes can have a key function; templates don't change that.
  non-template classes when computing the key function.
  - We always mark all of the virtual member functions of class
  template instantiations.
  - The vtable for an instantiation of a class template has weak
  linkage.

We could probably use available_externally linkage for vtables of
classes instantiated by explicit instantiation declarations (extern
templates), but GCC doesn't do this and I'm not 100% that the ABI
permits it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@92753 91177308-0d34-0410-b5e6-96231b3b80d8