[PDB] Use the raw PDB symbol interface more accurately

Summary:
This patch adds some symbol tag checks before using the `IPDBRawSymbol`
interface to improve safety and readability.

Reviewers: zturner

Reviewed By: zturner

Subscribers: lldb-commits, stella.stamenova

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D51967

llvm-svn: 342208