[clangd][NFC] Use std::string::replace in SourceCode:applyChange.

[njames93]

Summary

Just looks nicer and easier to read.

[kadircet]

thanks, lgtm!

clang-tools-extra/clangd/SourceCode.cpp

ssize_t ComputedRangeLength =
       lspLength(Contents.substr(*StartIndex, *EndIndex - *StartIndex));
 
   if (Change.rangeLength && ComputedRangeLength != *Change.rangeLength)
     return error(llvm::errc::invalid_argument,
                  "Change's rangeLength ({0}) doesn't match the "
                  "computed range length ({1}).",
                  *Change.rangeLength, ComputedRangeLength);
 
-  std::string NewContents;
-  NewContents.reserve(*StartIndex + Change.text.length() +
-                      (Contents.length() - *EndIndex));
-
-  NewContents = Contents.substr(0, *StartIndex);
-  NewContents += Change.text;
-  NewContents += Contents.substr(*EndIndex);
+  Contents.replace(*StartIndex, *EndIndex - *StartIndex, Change.text);
 
-  std::swap(Contents, NewContents);
   return llvm::Error::success();
 }
 
 EligibleRegion getEligiblePoints(llvm::StringRef Code,
                                  llvm::StringRef FullyQualifiedName,
                                  const LangOptions &LangOpts) {
   EligibleRegion ER;
   // Start with global namespace.