[codeview] support emitting indirect virtual base class information

Summary:
Fixes PR28281.

MSVC lists indirect virtual base classes in the field list of a class,
using LF_IVBCLASS records. This change makes LLVM emit such records
when processing DW_TAG_inheritance tags with the DIFlagVirtual and
(newly introduced) DIFlagIndirect tags.

Reviewers: rnk, ruiu, zturner

Differential Revision: https://reviews.llvm.org/D25578

llvm-svn: 285130