[clang-format] Fix misaligned trailing comments in the presence of an empty block comment.

Fixes https://github.com/llvm/llvm-project/issues/53441.

Expected code:
```
/**/   //
int a; //
```

was before misformatted to:
```
/**/     //
int a; //
```

Because the "remaining length" (after the starting `/*`) of an empty block comment `/**/` was computed to be 0 instead of 2.

Reviewed By: MyDeveloperDay, HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D118475

clang/lib/Format/BreakableToken.cpp

@@ -555,7 +555,9 @@ unsigned BreakableBlockComment::getRemainingLength(unsigned LineIndex,
     // We never need a decoration when breaking just the trailing "*/" postfix.
     bool HasRemainingText = Offset < Content[LineIndex].size();
     if (!HasRemainingText) {
-      LineLength -= Decoration.size();
+      bool HasDecoration = Lines[LineIndex].ltrim().startswith(Decoration);
+      if (HasDecoration)
+        LineLength -= Decoration.size();
     }
   }
   return LineLength;

clang/unittests/Format/FormatTestComments.cpp

@@ -2842,6 +2842,12 @@ TEST_F(FormatTestComments, AlignTrailingComments) {
              "#define FOO_NODELOCAL 4  // Loopback\n\n"
              "} // namespace m\n",
              getLLVMStyleWithColumns(80)));
+
+  // https://llvm.org/PR53441
+  verifyFormat("/* */  //\n"
+               "int a; //\n");
+  verifyFormat("/**/   //\n"
+               "int a; //\n");
 }
 
 TEST_F(FormatTestComments, AlignsBlockCommentDecorations) {