[mach-o] Support data-only dylibs

In final linked shared images, the __TEXT segment contains both code and
the mach-o header/load-commands.  In the case of a data-only dylib, there is
no code, so we need to force the addition of the __TEXT segment.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220597 91177308-0d34-0410-b5e6-96231b3b80d8
[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