[clang][AST][DeclPrinter] Change "__attribute__" AST printing location for the sake of GCC compatibility.

[mstachyraa]

Summary

When printing C representation of AST, clang puts the "attribute" elements after the name of printed function.
In order for the generated code to be GCC compatible, these attributes should be printed at the beginning of function.

For example when executing clang -cc1 -ast-print test.c we recieve:
void test() __attribute__((cold)) {}

Instead we should see:
__attribute__((cold))void test() {}
because the former causes GCC incompatibility

warning: GCC does not allow 'cold' attribute in this position on a function definition [-Wgcc-compat]

clang/lib/AST/DeclPrinter.cpp

 //===--- DeclPrinter.cpp - Printing implementation for Decl ASTs ----------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
void DeclPrinter::VisitFunctionDecl(FunctionDecl *D) {
 
   if (D->isFunctionTemplateSpecialization())
     Out << "template<> ";
   else if (!D->getDescribedFunctionTemplate()) {
     for (unsigned I = 0, NumTemplateParams = D->getNumTemplateParameterLists();
          I < NumTemplateParams; ++I)
       printTemplateParameters(D->getTemplateParameterList(I));
   }
   
+  prettyPrintAttributes(D);
+
   CXXConstructorDecl *CDecl = dyn_cast<CXXConstructorDecl>(D);
   CXXConversionDecl *ConversionDecl = dyn_cast<CXXConversionDecl>(D);
   CXXDeductionGuideDecl *GuideDecl = dyn_cast<CXXDeductionGuideDecl>(D);
   if (!Policy.SuppressSpecifiers) {
     switch (D->getStorageClass()) {
     case SC_None: break;
     case SC_Extern: Out << "extern "; break;
     case SC_Static: Out << "static "; break;
if (Expr *TrailingRequiresClause = D->getTrailingRequiresClause()) {
       Out << " requires ";
       TrailingRequiresClause->printPretty(Out, nullptr, SubPolicy, Indentation,
                                           "\n", &Context);
     }
   } else {
     Ty.print(Out, Policy, Proto);
   }
 
-  prettyPrintAttributes(D);
-
   if (D->isPure())
     Out << " = 0";
   else if (D->isDeletedAsWritten())
     Out << " = delete";
   else if (D->isExplicitlyDefaulted())
     Out << " = default";
   else if (D->doesThisDeclarationHaveABody()) {
     if (!Policy.TerseOutput) {