MS ABI: Get linkage of RTTI data correct

The Itanium rules are not appropriate for the MS ABI.  RTTI data is
_never_ imported and thus is never available_externally.  It is either
internal (if the type's linkage is internal) or linkonce_odr.

This also means that classes which inherit from dllimport'd bases force
their translation unit to duplicate the entirety of the RTTI data of
that base.

Interestingly, the complete object locator can never be referenced by
translation units which import the class.

This fixes PR20106.