Introduce NativeRawSymbol for PDB reading.

Introduce NativeRawSymbol for PDB reading.

This is a stub for a new concrete implementation of IPDBRawSymbol.
Nothing uses this uses this implementation yet.  My plan is to
locally switch lldb-pdbdump from the DIA reader to the Native one
and flesh out the implementations of these method stubs in the order
they're needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294633 91177308-0d34-0410-b5e6-96231b3b80d8