c++: fix propagating REF_PARENTHESIZED_P [PR116379]

c++: fix propagating REF_PARENTHESIZED_P [PR116379]

Here we have:

  template<typename T>
  struct X{
      T val;
      decltype(auto) value(){
	  return (val);
      }
  };

where the return type of value should be 'int &' since '(val)' is an
expression, not a name, and decltype(auto) performs the type deduction
using the decltype rules.

The problem is that we weren't propagating REF_PARENTHESIZED_P
correctly: the return value of finish_non_static_data_member in this
test was a REFERENCE_REF_P, so we didn't set the flag.  We should
use force_paren_expr like below.

	PR c++/116379

gcc/cp/ChangeLog:

	* pt.cc (tsubst_expr) <COMPONENT_REF>: Use force_paren_expr to set
	REF_PARENTHESIZED_P.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/decltype-auto9.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>