c++: Some preparatory type canonicalization fixes

The patches that follow revealed some latent type canonicalization
issues during normalization/satisfaction, due to normalization of
some constraint-expressions now being performed in different contexts
and more frequently (since the normalization caches get frequently
emptied in a checking compiler).  The issues are:

1. In tsubst_parameter_mapping, we canonicalize the arguments of a
   substituted TYPE_ARGUMENT_PACK only if the argument we started with
   wasn't a TYPE_ARGUMENT_PACK.  We should canonicalize a substituted
   TYPE_ARGUMENT_PACK regardless of what we started with.

2. We currently set DECL_CONTEXT and CONSTRAINT_VAR_P on each of the
   parameters introduced in a requires expression _after_ we're done
   processing the requirements.  But meanwhile we may have already
   built and computed the canonical form of a type that uses one of
   these PARM_DECLs (as say an operand to decltype).  But the canonical
   form depends on the result of cp_tree_equal, which in turn depends on
   the value of CONSTRAINT_VAR_P and DECL_CONTEXT.  So we must set these
   fields earlier, before processing requirements.

3. In do_auto_deduction, we use the result of finish_decltype_type later
   as a template argument, so we should canonicalize the result too.
   (While we're here, we should pass 'complain' to finish_decltype_type,
   which fixes the testcase auto1.C below.)

gcc/cp/ChangeLog:

	* constraint.cc (tsubst_parameter_mapping): Canonicalize the
	arguments of a substituted TYPE_ARGUMENT_PACK even if we've
	started with a TYPE_ARGUMENT_PACK.
	(finish_requires_expr): Don't set DECL_CONTEXT and
	CONSTRAINT_VAR_P on each of the introduced parameters here.
	* parser.c (cp_parser_requirement_parameter_list): Instead set
	these fields earlier, here.
	* pt.c (do_auto_deduction): Canonicalize the result of
	do_auto_deduction.  Pass 'complain' to finish_decltype_type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/auto1.C: New test.

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