[PDB][NativeSession] Clean up some things in NativeSession.

-Use the actual sect/offset to keep track of symbols in the cache so they don't get created multiple times with different addresses.
-Remove getSymTag from PDBFunctionSymbol/PDBPublicSymbol because it's already implemented in the base class
-Merge the symbolizer test files for DIA and native, since the tests are the same.
-Implement getCompilandId for NativeLineNumber

Reviewed By: amccarth

Differential Revision: https://reviews.llvm.org/D84208