[PDB] Add exclusive methods to derived symbol class

Summary: This commit adds two methods to the PDBSymboFunc class used in parsing symbols. getLineNumbers() is used to determine a Function symbol's declaration and getCompilandId() is used to initialize the SymbolContext field sc.comp_unit.

Reviewers: zturner, rnk, llvm-commits

Reviewed By: zturner

Differential Revision: https://reviews.llvm.org/D44457

llvm-svn: 327909