[clang] NFC: Use flush() idiomatically

[kbobyrev]

Summary

Using both raw_ostream::flush() and raw_ostream::str() consecutively is
redundant. The alternatives are:

• Use raw_ostream::str() without raw_ostream::flush()
• Use raw_ostream::flush() and then use the destination for raw_ostream writer

The latter is more idiomatic, so the fix resolves this particular case in its
favor.

clang/lib/AST/DeclPrinter.cpp

static void printExplicitSpecifier(ExplicitSpecifier ES, llvm::raw_ostream &Out,
   if (ES.getExpr()) {
     EOut << "(";
     ES.getExpr()->printPretty(EOut, nullptr, Policy, Indentation, "\n",
                               &Context);
     EOut << ")";
   }
   EOut << " ";
   EOut.flush();
-  Out << EOut.str();
+  Out << Proto;
 }
 
 void DeclPrinter::VisitFunctionDecl(FunctionDecl *D) {
   if (!D->getDescribedFunctionTemplate() &&
       !D->isFunctionTemplateSpecialization())
     prettyPrintPragmas(D);
 
   if (D->isFunctionTemplateSpecialization())