[llvm][GraphWriter] Resize std::string, instead of reassigning to substr (NFC)

* Don't call substr which creates a new string instance
* Only call string method if string length is larger than 140

Closes #90324

Author: JOE1994

llvm/lib/Support/GraphWriter.cpp

@@ -115,7 +115,8 @@ std::string llvm::createGraphFilename(const Twine &Name, int &FD) {
 
   // Windows can't always handle long paths, so limit the length of the name.
   std::string N = Name.str();
-  N = N.substr(0, std::min<std::size_t>(N.size(), 140));
+  if (N.size() > 140)
+    N.resize(140);
 
   // Replace illegal characters in graph Filename with '_' if needed
   std::string CleansedName = replaceIllegalFilenameChars(N, '_');