c++: Allow constexpr decltype(auto) [PR102229]

My r11-2202 was trying to enforce [dcl.type.auto.deduct]/4, which says
"If the placeholder-type-specifier is of the form type-constraint[opt]
decltype(auto), T shall be the placeholder alone."  But this made us
reject 'constexpr decltype(auto)', which, after clarification from CWG,
should be valid.  [dcl.type.auto.deduct]/4 is supposed to be a syntactic
constraint, not semantic, so it's OK that the constexpr marks the object
as const.

As a consequence, checking TYPE_QUALS in do_auto_deduction is too late,
and we have a FIXME there anyway.  So in this patch I'm attempting to
detect 'const decltype(auto)' earlier.  If I'm going to use TYPE_QUALS,
it needs to happen before we mark the object as const due to constexpr,
that is, before grokdeclarator's

  /* A `constexpr' specifier used in an object declaration declares
     the object as `const'.  */
  if (constexpr_p && innermost_code != cdk_function)
    ...

Constrained decltype(auto) was a little problem, hence the TYPENAME
check.  But in a typename context you can't use decltype(auto) anyway,
I think.

	PR c++/102229

gcc/cp/ChangeLog:

	* decl.c (check_decltype_auto): New.
	(grokdeclarator): Call it.
	* pt.c (do_auto_deduction): Don't check decltype(auto) here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/decltype-auto5.C: New test.