[codeview] emit debug info for indirect virtual base classes

Summary:
Fixes PR28281.

MSVC lists indirect virtual base classes in the field list of a class.
This change makes Clang emit the information necessary for LLVM to
emit such records.

Reviewers: rnk, ruiu, zturner

Differential Revision: https://reviews.llvm.org/D25579

llvm-svn: 285132