Xcode: use ranged for loops, cleanup existing for loops

The changes are mostly converting old-style explicit iterator for loops into ranged for statements. A number of for loops had already been changed over, but local variables had been left behind instead of being absorbed into the ranged for statement, so these have been cleaned up too. A couple of minor improvements were made in areas already being updated by the for loop changes to slightly simplify the code or to avoid unnecessary conversions between const char* and std::string.