c++: Fix decltype(auto) deduction with rvalue ref [PR78209]

c++: Fix decltype(auto) deduction with rvalue ref [PR78209]

Here's a small deficiency in decltype(auto).  [dcl.type.auto.deduct]/5:
If the placeholder-type-specifier is of the form decltype(auto), [...]
the type deduced for T is determined [...] as though E had been the operand
of the decltype.  So:

  int &&i = 0;
  decltype(auto) j = i; // should behave like int &&j = i; error

We deduce j's type in do_auto_deduction via finish_decltype_type which
takes an 'id' argument.  Currently we compute 'id' as false, because
stripped_init is *i (a REFERENCE_REF_P).  But it seems to me we should
rather set 'id' to true here, by looking through the REFERENCE_REF_P,
so that finish_decltype_type DTRT.

gcc/cp/ChangeLog:

	PR c++/78209
	* pt.c (do_auto_deduction): If init is REFERENCE_REF_P, use its
	first operand.

gcc/testsuite/ChangeLog:

	PR c++/78209
	* g++.dg/cpp1y/decltype-auto1.C: New test.