[lldb][gnustep] Add minimal GNUstepObjCRuntime plugin for LanguageTypeObjC on non-Apple platforms

This is the next patch after D146058. We can now parse expressions to print instance variables from ObjC classes. Until now the expression parser would bail out with an error like this:
```
error: expression failed to parse:
error: Error [IRForTarget]: Couldn't find Objective-C indirect ivar symbol OBJC_IVAR_$_TestObj._int
```

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D146154