add AR to acronyms of clang-tidy property check

[stephanemoore]

clang-tidy/objc/PropertyDeclarationCheck.cpp

37–38

This comment does not seem to be accurate anymore. The acronyms in this list are not all from the linked Apple documentation.

Sent out https://reviews.llvm.org/D46922.

45

Are we sure it's sustainable to keep growing this list of acronyms? I suspect there may be enough domain-specific acronyms that it might be better to accept any sequence of capitalized alphanumeric characters as an acronym.

clang-tidy/objc/PropertyDeclarationCheck.cpp

 // For CategoryProperty especially in categories of system class,
 // to avoid naming conflict, the suggested naming style is
 // 'abc_lowerCamelCase' (adding lowercase prefix followed by '_').
 enum NamingStyle {
   StandardProperty = 1,
   CategoryProperty = 2,
 };
 
 /// The acronyms are from
 /// https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CodingGuidelines/Articles/APIAbbreviations.html#//apple_ref/doc/uid/20001285-BCIHCGAE
 ///
 /// Keep this list sorted.
 constexpr llvm::StringLiteral DefaultSpecialAcronyms[] = {
     "[2-9]G",
     "ACL",
     "API",
+    "AR",
     "ARGB",
     "ASCII",
     "BGRA",
     "CA",
     "CF",
     "CG",
     "CI",
     "CV",