[siloptimizer] Use .str() on StringRef.

llvm recently removed the conversion operator that would convert
StringRef into str() implictly.  Now we have to be more explicit.
See: llvm/llvm-project@adcd026

Author: Matt Davis

lib/SILOptimizer/Utils/Differentiation/Common.cpp

@@ -357,7 +357,7 @@ SILDifferentiabilityWitness *getOrCreateMinimalASTDifferentiabilityWitness(
   if (!minimalConfig)
     return nullptr;
 
-  std::string originalName = original->getName();
+  std::string originalName = original->getName().str();
   // If original function requires a foreign entry point, use the foreign SIL
   // function to get or create the minimal differentiability witness.
   if (requiresForeignEntryPoint(originalAFD)) {