Fixed a logic error in Module::ResolveSymbolContextForAddress(). Asking an address if its offet is greater than zero doesn't actually correctly tell us wether the address is section offset or not. A symbol could be the first symbol in a section and its offset can be zero. Also, a non-section offset lldb_private::Address can have a NULL section and calling GetOffset() will return the absolute address. To really test if an address is section offset clients should use Address::IsSectionOffset(). Also simplified the code that backs the address up by one to use the Address::Slide() function.