Merge pull request #33165 from nathawes/fix-string-conversion-issue-master-rebranch

[master-rebranch] Fix a StringRef -> std::string conversion failure.

Author: Nathan Hawes

lib/SILOptimizer/Transforms/StringOptimization.cpp

@@ -187,7 +187,7 @@ bool StringOptimization::optimizeStringAppend(ApplyInst *appendCall,
   // Replace lhs.append(rhs) with "lhs = lhs + rhs" if both lhs and rhs are
   // constant.
   if (lhsString.isConstant() && rhsString.isConstant()) {
-    std::string concat = lhsString.str;
+    std::string concat = lhsString.str.str();
     concat += rhsString.str;
     if (ApplyInst *stringInit = createStringInit(concat, appendCall)) {
       replaceAppendWith(appendCall, stringInit, /*copyNewValue*/ false);