[clang-format] Handle "// clang-format on" for SeparateDefinitionBlocks

[owenpan]

Summary

Fixes https://github.com/llvm/llvm-project/issues/63393.

clang/lib/Format/DefinitionBlockSeparator.cpp

const auto MayPrecedeDefinition = [&](const int Direction = -1) {
       assert(Direction >= -1);
       assert(Direction <= 1);
       const size_t OperateIndex = OpeningLineIndex + Direction;
       assert(OperateIndex < Lines.size());
       const auto &OperateLine = Lines[OperateIndex];
       if (LikelyDefinition(OperateLine))
         return false;
 
-      if (OperateLine->First->is(tok::comment))
+      if (const auto *Tok = OperateLine->First;
+          Tok->is(tok::comment) && !isClangFormatOn(Tok->TokenText)) {
         return true;
+      }
 
       // A single line identifier that is not in the last line.
       if (OperateLine->First->is(tok::identifier) &&
           OperateLine->First == OperateLine->Last &&
           OperateIndex + 1 < Lines.size()) {
         // UnwrappedLineParser's recognition of free-standing macro like
         // Q_OBJECT may also recognize some uppercased type names that may be
         // used as return type as that kind of macros, which is a bit hard to

clang/unittests/Format/DefinitionBlockSeparatorTest.cpp

TestKit = MakeUntouchTest("namespace Ns {\n",
                             "} // namespace\n", true);
   verifyFormat(TestKit.first, AlwaysStyle, TestKit.second);
   verifyFormat(TestKit.second, NeverStyle, removeEmptyLines(TestKit.second));
 }
 
 TEST_F(DefinitionBlockSeparatorTest, Always) {
   FormatStyle Style = getLLVMStyle();
   Style.SeparateDefinitionBlocks = FormatStyle::SDS_Always;
+
+  verifyFormat("// clang-format off\n"
+               "template<class T>\n"
+               "concept C = not A<S<T>>;\n"
+               "// clang-format on\n"
+               "\n"
+               "struct E {};",
+               Style);
+
   std::string Prefix = "namespace {\n";
   std::string Infix = "\n"
                       "// Enum test1\n"
                       "// Enum test2\n"
                       "enum Foo { FOO, BAR };\n"
                       "\n"
                       "/*\n"
                       "test1\n"