[clang-format] Don't rewrite the input file if already formatted

[owenpan]

Summary

If IntegerLiteralSeparator is set but the integer literals are already formatted, don't rewrite the input file.

Fixes https://github.com/llvm/llvm-project/issues/60651.

clang/lib/Format/IntegerLiteralSeparatorFixer.cpp

for (bool Skip = false; !Lex.LexFromRawLexer(Tok);) {
     if (IsBase2)
       DigitsPerGroup = Binary;
     else if (IsBase16)
       DigitsPerGroup = Hex;
     if (DigitsPerGroup > 0 && checkSeparator(Text, DigitsPerGroup))
       continue;
     if (Start > 0)
       Location = Location.getLocWithOffset(Start);
-    cantFail(Result.add(tooling::Replacement(SourceMgr, Location, Length,
-                                             format(Text, DigitsPerGroup))));
+    if (const auto &Formatted = format(Text, DigitsPerGroup);
+        Formatted != Text) {
+      cantFail(Result.add(
+          tooling::Replacement(SourceMgr, Location, Length, Formatted)));
+    }
   }
 
   return {Result, 0};
 }
 
 bool IntegerLiteralSeparatorFixer::checkSeparator(
     const StringRef IntegerLiteral, int DigitsPerGroup) const {
   assert(DigitsPerGroup > 0);