current translation unit as available_externally. 

This helps devirtualize the second example in PR3100, comment 18:

struct S { S() {}; virtual void xyzzy(); };
inline void foo(S *s) { s->xyzzy(); }
void bar() { S s; foo(&s); }

This involved four major changes:

1. In DefineUsedVTables, always mark virtual member functions as referenced for
   non-template classes and class template specializations.
2. In CodeGenVTables::ShouldEmitVTableInThisTU return true if optimizations are
   enabled, even if the key function is not implemented in this translation 
   unit. We don't ever do this for code compiled with -fapple-kext, because we
   don't ever want to devirtualize virtual member function calls in that case.
3. Give the correct linkage for vtables where the key function is not defined.
4. Update the linkage for RTTI structures when necessary.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124565 91177308-0d34-0410-b5e6-96231b3b80d8