[lld-macho] Support Obj-C symbols in order files

Obj-C symbols may have spaces and colons, which our previous order file
parser would be confused by. The order file format has made the very unfortunate
choice of using colons for its delimiters, which means that we have to use
heuristics to determine if a given colon is part of a symbol or not...

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D93567