Make NativeExeSymbol a concrete subclass of NativeRawSymbol [PDB]

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.

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

llvm-svn: 298005