[llvm-pdbdump] Display padding bytes on record layout

When dumping classes, show where padding occurs, and at the end of the
class print statistics about how many bytes total of padding exist in a
class.

Since PDB doesn't specifically contain information about padding, we have
to mimic this by sort of reversing a small portion of the record layout
algorithm (e.g. looking at offsets and sizes and trying to determine
whether something is part of the same field or a new field).

Differential Revision: https://reviews.llvm.org/D31800

llvm-svn: 299869