[clang-format] Fix a crash on AllowShortFunctionsOnASingleLine

[owenpan]

Summary

Fixes https://github.com/llvm/llvm-project/issues/55008.

clang/lib/Format/TokenAnnotator.h

~AnnotatedLine() {
     FormatToken *Current = First;
     while (Current) {
       Current->Children.clear();
       Current->Role.reset();
       Current = Current->Next;
     }
   }
 
+  bool isComment() const {
+    return First && First->is(tok::comment) && !First->getNextNonComment();
+  }
+
   /// \c true if this line starts with the given tokens in order, ignoring
   /// comments.
   template <typename... Ts> bool startsWith(Ts... Tokens) const {
     return First && First->startsSequence(Tokens...);
   }
 
   /// \c true if this line ends with the given tokens in reversed order,
   /// ignoring comments.

clang/lib/Format/UnwrappedLineFormatter.cpp

auto ShouldMergeShortFunctions = [this, &I, &NextLine, PreviousLine,
           return true;
 
         if (TheLine->Level != 0) {
           if (!PreviousLine)
             return false;
 
           // TODO: Use IndentTracker to avoid loop?
           // Find the last line with lower level.
-          auto J = I - 1;
-          if (!TheLine->InPPDirective) {
-            for (; J != AnnotatedLines.begin(); --J) {
-              if (!(*J)->InPPDirective && (*J)->Level < TheLine->Level)
-                break;
+          const AnnotatedLine *Line = nullptr;
+          for (auto J = I - 1; J >= AnnotatedLines.begin(); --J) {
+            assert(*J);
+            if (!(*J)->InPPDirective && !(*J)->isComment() &&
+                (*J)->Level < TheLine->Level) {
+              Line = *J;
             }
           }
 
-          if ((*J)->Level >= TheLine->Level)
+          if (!Line)
             return false;
 
           // Check if the found line starts a record.
-          const FormatToken *LastNonComment = (*J)->Last;
+          const FormatToken *LastNonComment = Line->Last;
           assert(LastNonComment);
           if (LastNonComment->is(tok::comment)) {
             LastNonComment = LastNonComment->getPreviousNonComment();
             // There must be another token (usually `{`), because we chose a
-            // line that has a smaller level.
+            // non-PPDirective and non-comment line that has a smaller level.
             assert(LastNonComment);
           }
           return isRecordLBrace(*LastNonComment);
         }
       }
 
       return false;
     };

clang/unittests/Format/FormatTest.cpp

TEST_F(FormatTest, PullInlineFunctionDefinitionsIntoSingleLine) {
 
   verifyFormat("class C {\n"
                "#ifdef A\n"
                "  int f() { return 42; }\n"
                "#endif\n"
                "};",
                MergeInlineOnly);
 
+  verifyFormat("struct S {\n"
+               "// comment\n"
+               "#ifdef FOO\n"
+               "  int foo() { bar(); }\n"
+               "#endif\n"
+               "};",
+               MergeInlineOnly);
+
   // Also verify behavior when BraceWrapping.AfterFunction = true
   MergeInlineOnly.BreakBeforeBraces = FormatStyle::BS_Custom;
   MergeInlineOnly.BraceWrapping.AfterFunction = true;
   verifyFormat("class C {\n"
                "  int f() { return 42; }\n"
                "};",
                MergeInlineOnly);
   verifyFormat("int f()\n"