Enable clang-provided niceties for Objective-C code

Clang supports Objective-C just as it supports C and C++, but objc was missing from the clang.plugin file, which meant there was no smart completion or other quality-of-life features.

Enabling objc in clang.plugin seems to be enough to make everything work. At least, now I do get completion:

There also exist Objective-C header files (.h) and Objective-C++ code (.mm) and header (.h, .hh, .hpp, etc) files. I don't think there are distinct language codes for them, at least not out of the box, so ¯\_(ツ)_/¯ how to make clang handle them as well.