[codeview] Emit base class information from DW_TAG_inheritance nodes

There are two remaining issues here:
1. No vbptr information
2. Need to mention indirect virtual bases

Getting indirect virtual bases is just a matter of adding an "indirect"
flag, emitting them in the frontend, and ignoring them when appropriate
for DWARF.

All virtual bases use the same artificial vbptr field, so I think the
vbptr offset will be best represented by an implicit __vbptr$ClassName
member similar to our existing __vptr$ member.

llvm-svn: 273688