[clang-format] Regroup #includes into blocks for Google style

[ioeric]

Summary

Regrouping #includes in blocks separated by blank lines when sorting C++ #include
headers was implemented recently, and it has been preferred in Google's C++ style guide:
https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes

[ioeric]

• Improved test.

lib/Format/Format.cpp

FormatStyle getGoogleStyle(FormatStyle::LanguageKind Language) {
   GoogleStyle.AllowShortLoopsOnASingleLine = true;
   GoogleStyle.AlwaysBreakBeforeMultilineStrings = true;
   GoogleStyle.AlwaysBreakTemplateDeclarations = FormatStyle::BTDS_Yes;
   GoogleStyle.ConstructorInitializerAllOnOneLineOrOnePerLine = true;
   GoogleStyle.DerivePointerAlignment = true;
   GoogleStyle.IncludeStyle.IncludeCategories = {
       {"^<ext/.*\\.h>", 2}, {"^<.*\\.h>", 1}, {"^<.*", 2}, {".*", 3}};
   GoogleStyle.IncludeStyle.IncludeIsMainRegex = "([-_](test|unittest))?$";
+  GoogleStyle.IncludeStyle.IncludeBlocks = tooling::IncludeStyle::IBS_Regroup;
   GoogleStyle.IndentCaseLabels = true;
   GoogleStyle.KeepEmptyLinesAtTheStartOfBlocks = false;
   GoogleStyle.ObjCBinPackProtocolList = FormatStyle::BPS_Never;
   GoogleStyle.ObjCSpaceAfterProperty = false;
   GoogleStyle.ObjCSpaceBeforeProtocolList = true;
   GoogleStyle.PointerAlignment = FormatStyle::PAS_Left;
   GoogleStyle.RawStringFormats = {
       {

unittests/Format/CleanupTest.cpp

tooling::Replacements Replaces = toReplacements(
        createInsertion("#include <vector>"), createInsertion("#include <list>"),
        createInsertion("#include \"fix.h\"")});
   EXPECT_EQ(Expected, formatAndApply(Code, Replaces));
 }
 
 TEST_F(CleanUpReplacementsTest, InsertMultipleNewHeadersAndSortGoogle) {
   std::string Code = "\nint x;";
   std::string Expected = "\n#include \"fix.h\"\n"
+                         "\n"
                          "#include <list>\n"
                          "#include <vector>\n"
+                         "\n"
                          "#include \"a.h\"\n"
                          "#include \"b.h\"\n"
                          "#include \"c.h\"\n"
                          "int x;";
   tooling::Replacements Replaces = toReplacements(
       {createInsertion("#include \"a.h\""), createInsertion("#include \"c.h\""),
        createInsertion("#include \"b.h\""),
        createInsertion("#include <vector>"), createInsertion("#include <list>"),

unittests/Format/SortIncludesTest.cpp

EXPECT_EQ("#include \"a.h\"\n"
                  "#include \"a.h\"\n"
                  "// comment\n"
                  "#include \"b.h\"\n"));
 }
 
 TEST_F(SortIncludesTest, HandlesAngledIncludesAsSeparateBlocks) {
   EXPECT_EQ("#include \"a.h\"\n"
             "#include \"c.h\"\n"
+            "#include <array>\n"
             "#include <b.h>\n"
-            "#include <d.h>\n",
-            sort("#include <d.h>\n"
+            "#include <d.h>\n"
+            "#include <vector>\n",
+            sort("#include <vector>\n"
+                 "#include <d.h>\n"
+                 "#include <array>\n"
                  "#include <b.h>\n"
                  "#include \"c.h\"\n"
                  "#include \"a.h\"\n"));
 
   FmtStyle = getGoogleStyle(FormatStyle::LK_Cpp);
   EXPECT_EQ("#include <b.h>\n"
             "#include <d.h>\n"
+            "\n"
+            "#include <array>\n"
+            "#include <vector>\n"
+            "\n"
             "#include \"a.h\"\n"
             "#include \"c.h\"\n",
-            sort("#include <d.h>\n"
+            sort("#include <vector>\n"
+                 "#include <d.h>\n"
+                 "#include <array>\n"
                  "#include <b.h>\n"
                  "#include \"c.h\"\n"
                  "#include \"a.h\"\n"));
 }
 
 TEST_F(SortIncludesTest, RegroupsAngledIncludesInSeparateBlocks) {
   Style.IncludeBlocks = tooling::IncludeStyle::IBS_Regroup;
   EXPECT_EQ("#include \"a.h\"\n"