This implements searching for function symbols and public symbols by address.
More specifically,
-Implements NativeSession::findSymbolByAddress for function symbols and public symbols. I think data symbols are also searched for, but that isn't implemented here.
-Adds classes for NativeFunctionSymbol and NativePublicSymbol
-Adds a '-use-native-pdb-reader' option to llvm-symbolizer, for testing
Another -FromAddress versus -FromSectOffset.