[PDB] Fix a bug in the explain subcommand.

We were trying to dig into the super block fields and print a
description of the field at the specified offset, but we were
printing the wrong field due to an off-by-one-field-error.

llvm-svn: 328804
GitOrigin-RevId: 1b20416bfa4ed906d5b5f289aa8a4430b9620132
2 files changed