[clang-tidy/checks] Implement a clang-tidy check to verify Google Objective-C function naming conventions 📜

Summary:
§1 Description

This check finds function names in function declarations in Objective-C files that do not follow the naming pattern described in the Google Objective-C Style Guide. Function names should be in UpperCamelCase and functions that are not of static storage class should have an appropriate prefix as described in the Google Objective-C Style Guide. The function `main` is a notable exception. Function declarations in expansions in system headers are ignored.

Example conforming function definitions:
```
static bool IsPositive(int i) { return i > 0; }
static bool ABIsPositive(int i) { return i > 0; }
bool ABIsNegative(int i) { return i < 0; }
```

A fixit hint is generated for functions of static storage class but otherwise the check does not generate a fixit hint because an appropriate prefix for the function cannot be determined.

§2 Test Notes
* Verified clang-tidy tests pass successfully.
* Used check_clang_tidy.py to verify expected output of processing google-objc-function-naming.m

Reviewers: benhamilton, hokein, Wizard, aaron.ballman

Reviewed By: benhamilton

Subscribers: Eugene.Zelenko, mgorny, xazax.hun, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D51575

llvm-svn: 347132

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