[NFC][Sema] Return underlying strings directly instead of OS.str()

This avoids an unnecessary copy required by 'return OS.str()', allowing
instead for NRVO or implicit move. The .str() call (which flushes the
stream) is no longer required since 65b1361,
which made raw_string_ostream unbuffered by default.

Differential Revision: https://reviews.llvm.org/D115374

clang/lib/Sema/CodeCompleteConsumer.cpp

@@ -335,7 +335,7 @@ std::string CodeCompletionString::getAsString() const {
       break;
     }
   }
-  return OS.str();
+  return Result;
 }
 
 const char *CodeCompletionString::getTypedText() const {
@@ -640,7 +640,7 @@ static std::string getOverloadAsString(const CodeCompletionString &CCS) {
       break;
     }
   }
-  return OS.str();
+  return Result;
 }
 
 void PrintingCodeCompleteConsumer::ProcessOverloadCandidates(

clang/lib/Sema/SemaAttr.cpp

@@ -792,7 +792,7 @@ attrMatcherRuleListToString(ArrayRef<attr::SubjectMatchRule> Rules) {
       OS << (I.index() == Rules.size() - 1 ? ", and " : ", ");
     OS << "'" << attr::getSubjectMatchRuleSpelling(I.value()) << "'";
   }
-  return OS.str();
+  return Result;
 }
 
 } // end anonymous namespace