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

[curdeius]

Summary

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.

clang/lib/Format/BreakableToken.cpp

unsigned BreakableBlockComment::getRemainingLength(unsigned LineIndex,
   unsigned LineLength =
       UnbreakableTailLength +
       getRangeLength(LineIndex, Offset, StringRef::npos, StartColumn);
   if (LineIndex + 1 == Lines.size()) {
     LineLength += 2;
     // We never need a decoration when breaking just the trailing "*/" postfix.
     bool HasRemainingText = Offset < Content[LineIndex].size();
     if (!HasRemainingText) {
+      bool HasDecoration = Lines[LineIndex].ltrim().startswith(Decoration);
+      if (HasDecoration)
         LineLength -= Decoration.size();
     }
   }
   return LineLength;
 }
 
 unsigned BreakableBlockComment::getContentStartColumn(unsigned LineIndex,
                                                       bool Break) const {
   if (Break)

clang/unittests/Format/FormatTestComments.cpp

EXPECT_EQ(
       format("namespace m {\n\n"
              "#define FOO_GLOBAL 0   // Global scope.\n"
              "#define FOO_LINKLOCAL 1  // Link-local scope.\n"
              "#define FOO_SITELOCAL 2  // Site-local scope (deprecated).\n"
              "#define FOO_UNIQUELOCAL 3 // Unique local\n"
              "#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) {
   EXPECT_EQ("/*\n"
             " */",
             format("/*\n"
                    "*/",
                    getLLVMStyle()));