[mach-o] Support old style objc class names in export lists

Objective-C switched to a new ABI which uses a different mangling for class
names.  But to keep projects building that use export lists that use the old
class name mangling, the linker recognizes the old names and transforms them
to the new mangling.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220598 91177308-0d34-0410-b5e6-96231b3b80d8