MS ABI: Reference MSVC RTTI from the VFTable

The pointer for a class's RTTI data comes right before the VFTable but
has no name.  To be properly compatible with this, we do the following:
* Create a single GlobalVariable which holds the contents of the VFTable
  _and_ the pointer to the RTTI data.
* Create a GlobalAlias, with appropriate linkage/visibility, that points
  just after the RTTI data pointer.  This ensures that the VFTable
  symbol will always refer to VFTable data.
* Create a Comdat with a "Largest" SelectionKind and stick the private
  GlobalVariable in it.  By transitivity, the GlobalAlias will be a
  member of the Comdat group.  Using "Largest" ensures that foreign
  definitions without an RTTI data pointer will _not_ be chosen in the
  final linked image.

Whether or not we emit RTTI data depends on several things:
* The -fno-rtti flag implies that we should never not emit a pointer to
  RTTI data before the VFTable.
* __declspec(dllimport) brings in the VFTable from a remote DLL. Use an
  available_externally GlobalVariable to provide a local definition of
  the VFTable.  This means that we won't have any available_externally
  definitions of things like complete object locators.  This is
  acceptable because they are never directly referenced.

To my knowledge, this completes the implementation of MSVC RTTI code
generation.

Further semantic work should be done to properly support /GR-.