[PDB] Add a method to get the full path of the source file for PDBSymbolCompiland

Summary:
Redefine PDBSymbolCompiland::getSourceFileName() to return the filename (w/o directory) of the source file that is used to compile the compiland. This is because the result returned previously is ambiguous. It could be the filename, relative path or full path of the source file. 

Move the implementation of SymbolFilePDB::GetSourceFileNameForPDBCompiland() into a new method PDBSymbolCompiland::getSourceFileFullPath(). 

Reviewers: zturner, rnk, llvm-commits

Reviewed By: zturner

Differential Revision: https://reviews.llvm.org/D44458

llvm-svn: 327910