[llvm-pdbdump] Re-write the record layout code to be more resilient.

This reworks the way virtual bases are handled, and also the way
padding is detected across multiple levels of aggregates, producing
a much more accurate result.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301203 91177308-0d34-0410-b5e6-96231b3b80d8
25 files changed