[clang-tidy ObjC] [1/3] New module `objc` for Objective-C checks

Summary:
This is part 1 of 3 of a series of changes to improve Objective-C
linting in clang-tidy.

This introduces a new clang-tidy module, `objc`, specifically for
Objective-C / Objective-C++ checks.

The module is currently empty; D39142 adds the first check.

Test Plan: `ninja check-clang-tools`

Patch by Ben Hamilton!

Reviewers: hokein, alexfh

Reviewed By: hokein

Subscribers: Wizard, mgorny

Differential Revision: https://reviews.llvm.org/D39188

llvm-svn: 316643