[NativePDB] Fix access to both old & new fpo data entries from dbi stream

Summary:
This patch fixes access to fpo streams in native pdb from DbiStream and makes
code consistent with DbiStreamBuilder.

Patch By: leonid.mashinskiy

Reviewers: zturner, aleksandr.urakov

Reviewed By: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56725

llvm-svn: 352615