MachoFile should default to using subsections_via_symbols.

When we parse a MachoFile, we set a number of members from the parsed
file, for example, subsectionsViaSymbols.

However, a number of passes, such as ObjCPass, create local copies of
MachoFile and don't get the benefit of setting flags and other fields in
the parser.  Instead we can just give a more sensible default as the parser
will definitely get the correct value from the file anyway.

llvm-svn: 259426