Make sure Objective-C category support in IncludeSorter handles top-level imports

[compositeprimes]

Summary

Currently, this would not correctly associate a category with the related include if it was top-level (i.e. no slashes in the path). This ensures that we explicitly think about that case.

[gribozavr2]

Any chance of adding a test?

[compositeprimes]

In D89608#2336596, @gribozavr2 wrote:

Any chance of adding a test?

Done!

clang-tools-extra/clang-tidy/utils/IncludeSorter.cpp

StringRef MakeCanonicalName(StringRef Str, IncludeSorter::IncludeStyle Style) {
   if (Style == IncludeSorter::IS_Google_ObjC) {
     StringRef Canonical =
         RemoveFirstSuffix(RemoveFirstSuffix(Str, {".cc", ".cpp", ".c", ".h",
                                                   ".hpp", ".mm", ".m"}),
                           {"_unittest", "_regtest", "_test", "Test"});
 
     // Objective-C categories have a `+suffix` format, but should be grouped
     // with the file they are a category of.
+    size_t start_index = Canonical.find_last_of('/');
+    if (start_index == StringRef::npos) {
+      start_index = 0;
+    }
     return Canonical.substr(
-        0, Canonical.find_first_of('+', Canonical.find_last_of('/')));
+        0, Canonical.find_first_of('+', start_index));
   }
   return RemoveFirstSuffix(
       RemoveFirstSuffix(Str, {".cc", ".cpp", ".c", ".h", ".hpp"}),
       {"_unittest", "_regtest", "_test"});
 }
 
 // Scan to the end of the line and return the offset of the next line.
 size_t FindNextLine(const char *Text) {

clang-tools-extra/unittests/clang-tidy/IncludeInserterTest.cpp

 std::string runCheckOnCode(StringRef Code, StringRef Filename) {
   std::vector<ClangTidyError> Errors;
   return test::runCheckOnCode<Check>(Code, &Errors, Filename, None,
                                      ClangTidyOptions(),
                                      {// Main file include
                                       {"clang_tidy/tests/"
                                        "insert_includes_test_header.h",
                                        "\n"},
+                                      // Top-level main file include +
+                                      // category.
+                                      {"top_level_test_header.h", "\n"},
+                                      {"top_level_test_header+foo.h", "\n"},
                                       // ObjC category.
                                       {"clang_tidy/tests/"
                                        "insert_includes_test_header+foo.h",
                                        "\n"},
                                       // Non system headers
                                       {"a/header.h", "\n"},
                                       {"path/to/a/header.h", "\n"},
                                       {"path/to/z/header.h", "\n"},
void foo() {
   EXPECT_EQ(
       PostCode,
       runCheckOnCode<ObjCEarlyInAlphabetHeaderInserterCheck>(
           PreCode, "repo/clang_tidy/tests/insert_includes_test_header.mm"));
 }
 
 TEST(IncludeInserterTest, InsertCategoryHeaderObjectiveC) {
   const char *PreCode = R"(
-#import "clang_tidy/tests/insert_includes_test_header.h"
+#import "top_level_test_header.h"
 
 void foo() {
   int a = 0;
 })";
   const char *PostCode = R"(
-#import "clang_tidy/tests/insert_includes_test_header.h"
-#import "clang_tidy/tests/insert_includes_test_header+foo.h"
+#import "top_level_test_header.h"
+#import "top_level_test_header+foo.h"
 
 void foo() {
   int a = 0;
 })";
 
   EXPECT_EQ(
       PostCode,
       runCheckOnCode<ObjCCategoryHeaderInserterCheck>(
-          PreCode, "repo/clang_tidy/tests/insert_includes_test_header.mm"));
+          PreCode, "top_level_test_header.mm"));
 }
 
 TEST(IncludeInserterTest, InsertGeneratedHeaderObjectiveC) {
   const char *PreCode = R"(
 #import "clang_tidy/tests/insert_includes_test_header.h"
 
 #include <list>
 #include <map>