[mach-o] switch to use llvm::MachO:: constants

Stop using some locally defined mach-o constants.



git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191581 91177308-0d34-0410-b5e6-96231b3b80d8
Add a comment to ReaderCOFF::parseFile.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191583 91177308-0d34-0410-b5e6-96231b3b80d8