[ms-cxxabi] Emit linkonce complete dtors in TUs that need them

Based on Peter Collingbourne's destructor patches.

Prior to this change, clang was considering ?1 to be the complete
destructor and the base destructor, which was wrong.  This lead to
crashes when clang tried to emit two LLVM functions with the same name.

In this ABI, TUs with non-inline dtors might not emit a complete
destructor.  They are emitted as inline thunks in TUs that need them,
and they always delegate to the base dtors of the complete class and its
virtual bases.  This change uses the DeferredDecls machinery to emit
complete dtors as needed.

Currently in clang try body destructors can catch exceptions thrown by
virtual base destructors.  In the Microsoft C++ ABI, clang may not have
the destructor definition, in which case clang won't wrap the virtual
virtual base destructor calls in a try-catch.  Diagnosing this in user
code is TODO.

Finally, for classes that don't use virtual inheritance, MSVC always
calls the base destructor (?1) directly.  This is a useful code size
optimization that avoids emitting lots of extra thunks or aliases.
Implementing it also means our existing tests continue to pass, and is
consistent with MSVC's output.

We can do the same for Itanium by tweaking GetAddrOfCXXDestructor, but
it will require further testing.

Reviewers: rjmccall

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1066