Re-land: "Make NativeExeSymbol a concrete subclass of NativeRawSymbol [PDB]"

This should work on all platforms now that r299006 has landed.  Tested locally
on Windows and Linux.

This moves exe symbol-specific method implementations out of NativeRawSymbol
into a concrete subclass. Also adds implementations for hasCTypes and
hasPrivateSymbols and a simple test to ensure the native reader can access the
summary information for the executable from the PDB.

Original Differential Revision: https://reviews.llvm.org/D31059

llvm-svn: 299019