[typechecker] Convert a std::string -> SmallString.

Just noticed this as I was reading the code.

Author: gottesmm

lib/Sema/TypeCheckConcurrency.cpp

@@ -1386,9 +1386,9 @@ void swift::diagnoseMissingExplicitSendable(NominalTypeDecl *nominal) {
       // based on the requirements harvested from instance storage.
 
       // Form the where clause containing all of the requirements.
-      std::string whereClause;
+      SmallString<64> whereClause;
       {
-        llvm::raw_string_ostream out(whereClause);
+        llvm::raw_svector_ostream out(whereClause);
         llvm::interleaveComma(
             requirements, out,
             [&](const Requirement &req) {