DeclPrinter: fix CXXConstructExpr printing with implicit default argument

This is an improvement of r173630, that handles the following case:

  struct VirualDestrClass
  {
    VirualDestrClass(int arg);
    virtual ~VirualDestrClass();
  };

  struct ConstrWithCleanupsClass
  {
    ConstrWithCleanupsClass(const VirualDestrClass& cplx = VirualDestrClass(42));
  };

  ConstrWithCleanupsClass cwcNoArg;

That was printed as:

  ConstrWithCleanupsClass cwcNoArg();

llvm-svn: 174296