[Frontend] Use SmallString::operator std::string (NFC)

Author: kazutakahirata

flang/lib/Frontend/CompilerInstance.cpp

@@ -80,7 +80,7 @@ static std::string getOutputFilePath(llvm::StringRef outputFilename,
   if (!extension.empty() && (inputFilename != "-")) {
     llvm::SmallString<128> path(inputFilename);
     llvm::sys::path::replace_extension(path, extension);
-    outFile = std::string(path.str());
+    outFile = std::string(path);
   }
 
   return outFile;

flang/lib/Frontend/TextDiagnosticBuffer.cpp

@@ -35,20 +35,20 @@ void TextDiagnosticBuffer::HandleDiagnostic(
     llvm_unreachable("Diagnostic not handled during diagnostic buffering!");
   case clang::DiagnosticsEngine::Note:
     all.emplace_back(level, notes.size());
-    notes.emplace_back(info.getLocation(), std::string(buf.str()));
+    notes.emplace_back(info.getLocation(), std::string(buf));
     break;
   case clang::DiagnosticsEngine::Warning:
     all.emplace_back(level, warnings.size());
-    warnings.emplace_back(info.getLocation(), std::string(buf.str()));
+    warnings.emplace_back(info.getLocation(), std::string(buf));
     break;
   case clang::DiagnosticsEngine::Remark:
     all.emplace_back(level, remarks.size());
-    remarks.emplace_back(info.getLocation(), std::string(buf.str()));
+    remarks.emplace_back(info.getLocation(), std::string(buf));
     break;
   case clang::DiagnosticsEngine::Error:
   case clang::DiagnosticsEngine::Fatal:
     all.emplace_back(level, errors.size());
-    errors.emplace_back(info.getLocation(), std::string(buf.str()));
+    errors.emplace_back(info.getLocation(), std::string(buf));
     break;
   }
 }