[clang-reorder-fields] Reorder leading comments (#123740)

Similarly to https://github.com/llvm/llvm-project/pull/122918, leading
comments are currently not being moved.

```
struct Foo {
  // This one is the cool field.
  int a;
  int b;
};
```

becomes:

```
struct Foo {
  // This one is the cool field.
  int b;
  int a;
};
```

but should be:

```
struct Foo {
  int b;
  // This one is the cool field.
  int a;
};
```

1 files changed, 35 insertions, 0 deletions

diff --git a/clang/unittests/Lex/LexerTest.cpp b/clang/unittests/Lex/LexerTest.cpp
index c897998..29c61fe 100644
--- a/clang/unittests/Lex/LexerTest.cpp
+++ b/clang/unittests/Lex/LexerTest.cpp
@@ -640,6 +640,41 @@ TEST_F(LexerTest, FindNextTokenIncludingComments) {
                           "=", "abcd", ";"));
 }
 
+TEST_F(LexerTest, FindPreviousToken) {
+  Lex("int abcd = 0;\n"
+      "// A comment.\n"
+      "int xyz = abcd;\n");
+  std::vector<std::string> GeneratedByPrevToken;
+  SourceLocation Loc = SourceMgr.getLocForEndOfFile(SourceMgr.getMainFileID());
+  while (true) {
+    auto T = Lexer::findPreviousToken(Loc, SourceMgr, LangOpts, false);
+    if (!T.has_value())
+      break;
+    GeneratedByPrevToken.push_back(getSourceText(*T, *T));
+    Loc = Lexer::GetBeginningOfToken(T->getLocation(), SourceMgr, LangOpts);
+  }
+  EXPECT_THAT(GeneratedByPrevToken, ElementsAre(";", "abcd", "=", "xyz", "int",
+                                                ";", "0", "=", "abcd", "int"));
+}
+
+TEST_F(LexerTest, FindPreviousTokenIncludingComments) {
+  Lex("int abcd = 0;\n"
+      "// A comment.\n"
+      "int xyz = abcd;\n");
+  std::vector<std::string> GeneratedByPrevToken;
+  SourceLocation Loc = SourceMgr.getLocForEndOfFile(SourceMgr.getMainFileID());
+  while (true) {
+    auto T = Lexer::findPreviousToken(Loc, SourceMgr, LangOpts, true);
+    if (!T.has_value())
+      break;
+    GeneratedByPrevToken.push_back(getSourceText(*T, *T));
+    Loc = Lexer::GetBeginningOfToken(T->getLocation(), SourceMgr, LangOpts);
+  }
+  EXPECT_THAT(GeneratedByPrevToken,
+              ElementsAre(";", "abcd", "=", "xyz", "int", "// A comment.", ";",
+                          "0", "=", "abcd", "int"));
+}
+
 TEST_F(LexerTest, CreatedFIDCountForPredefinedBuffer) {
   TrivialModuleLoader ModLoader;
   auto PP = CreatePP("", ModLoader);