Emit available_externally vtables opportunistically

Summary:
We can emit vtable definition having inline function
if they are all emitted.

Reviewers: rjmccall, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33437

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304394 91177308-0d34-0410-b5e6-96231b3b80d8