Support ObjC in IncludeInserter

[compositeprimes]

Summary

Update IncludeSorter/IncludeInserter to support objective-c google style (part 1):

1. Correctly consider .mm/.m extensions
2. Correctly categorize category headers.
3. Add support for generated files to go in a separate section of imports

[compositeprimes]

clang-format is complaining about my ordering of headers in my test case in IncludeInserterTest (but that's correct objc formatting). Not sure if there's a way to disable that on those lines

[alexfh]

LG

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

 #import "clang_tidy/tests/insert_includes_test_header.h"
 #import "clang_tidy/tests/insert_includes_test_header+foo.h"
 
 void foo() {
   int a = 0;
 })";
 
   EXPECT_EQ(PostCode, runCheckOnCode<ObjCCategoryHeaderInserterCheck>(
-                          PreCode, "devtools/cymbal/clang_tidy/tests/"
+                          PreCode, "repo/clang_tidy/tests/"
                                    "insert_includes_test_header.mm"));
 }
 
 TEST(IncludeInserterTest, InsertGeneratedHeaderObjectiveC) {
   const char *PreCode = R"(
 #import "clang_tidy/tests/insert_includes_test_header.h"
 
 #include <list>

 
 #import "clang_tidy/tests/generated_file.proto.h"
 
 void foo() {
   int a = 0;
 })";
 
   EXPECT_EQ(PostCode, runCheckOnCode<ObjCGeneratedHeaderInserterCheck>(
-                          PreCode, "devtools/cymbal/clang_tidy/tests/"
+                          PreCode, "repo/clang_tidy/tests/"
                                    "insert_includes_test_header.mm"));
 }
 
 } // anonymous namespace
 } // namespace tidy
 } // namespace clang
 
 #endif