Support Objective-C/C++ source files in check_clang_tidy.py

check_clang_tidy.py currently only handles C and C++ source files.

This extends the logic to also handle Objective-C (.m) and
Objective-C++ (.mm) files.

However, by default, clang compiles .m/.mm files using Objective-C 1.0
syntax. Objective-C 2.0 has been the default in Xcode for about 10
years, and Objective-C Automatic Reference Counting (ARC) for about 6
years, so this enables both by default.

(Clients which actually want to test clang-tidy checks for Objective-C
 1.0 or non-ARC files can pass custom flags to check_clang_tidy.py
 after --, which will disable the Objective-C 2.0 and ARC flags).

I did not add logic to handle running clang-tidy on Objective-C header
files alone; they also use the .h file extension, so we'd need to
look inside their contents.

I included a new test to confirm the new behavior.

Depends On D38963

Patch by Ben Hamilton!

llvm-svn: 316090