[clang-tidy/google] Improve the Objective-C global variable declaration check 🔧

Summary:
The current Objective-C global variable declaration check restricts naming that is permitted by the Google Objective-C style guide.

The Objective-C style guide states the following:
"Global and file scope constants should have an appropriate prefix. [...] Constants may use a lowercase k prefix when appropriate"
http://google.github.io/styleguide/objcguide#constants

This change fixes the check to allow two or more capital letters as an appropriate prefix. This change intentionally avoids making a decision regarding whether to flag constants that use a two letter prefix (two letter prefixes are reserved by Apple¹ but many projects seem to violate this guideline).

This change eliminates an important category of false positives (constants prefixed with '[A-Z]{2,}') at the cost of introducing a less important category of false negatives (constants prefixed with only '[A-Z]'). The false positives are observed in standard recommended code while the false negatives occur in non-standard unrecommended code. The number of eliminated false positives is expected to be significantly larger than the number of exposed false negatives.

❧

(1)
"Two-letter prefixes like these are reserved by Apple for use in framework classes."
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Conventions/Conventions.html

Reviewers: aaron.ballman, Wizard, hokein, benhamilton

Reviewed By: aaron.ballman, Wizard

Subscribers: aaron.ballman, cfe-commits

Differential Revision: https://reviews.llvm.org/D43581

llvm-svn: 326046