Allow IncludeSorter to use #import for Objective-C files

[compositeprimes]

Summary

The change infers whether a source file is Objective-C using LangOpts, and if it detects Objective-C, it uses "#import" instead of "#include" for all inserted imports.

[hokein]

Please add a unittest for it, you can add it in unittests/clang-tidy/IncludeInserterTest.cpp.

clang-tidy/utils/IncludeSorter.cpp

116

What about the ObjC++? The current behavior is always using #import, google objc style guide says #import Objective-C and Objective-C++ headers, and #include C/C++ headers., I don't think we have a way to do it smartly.

Maybe a conservative way is to use #import for ObjC only?

[benhamilton]

clang-tidy/utils/IncludeSorter.cpp

116

Yeah, came here to say this. Let's enable #import for ObjC but disable for C++ (which will include ObjC++).

clang-tidy/utils/IncludeSorter.cpp

 
 Optional<FixItHint> IncludeSorter::CreateIncludeInsertion(StringRef FileName,
                                                           bool IsAngled) {
+  std::string IncludeDirective = LangOpts->ObjC ? "#import " : "#include ";
   std::string IncludeStmt =
-      IsAngled ? llvm::Twine("#include <" + FileName + ">\n").str()
-               : llvm::Twine("#include \"" + FileName + "\"\n").str();
+      IsAngled ? llvm::Twine(IncludeDirective + "<" + FileName + ">\n").str()
+               : llvm::Twine(IncludeDirective + "\"" + FileName + "\"\n").str();
   if (SourceLocations.empty()) {
     // If there are no includes in this file, add it in the first line.
     // FIXME: insert after the file comment or the header guard, if present.