c++/modules: Stream all REQUIRES_EXPR_PARMS [PR122789]

c++/modules: Stream all REQUIRES_EXPR_PARMS [PR122789]

We don't generally stream the TREE_CHAIN of a DECL, as this would cause
us to unnecessarily walk into the next member in its scope chain any
time it was referenced by an expression.

Unfortunately, REQUIRES_EXPR_PARMS is a tree chain of PARM_DECLs, so we
were only ever streaming the first parameter.  This meant that when a
parameter's type could not be tsubst'd we would ICE instead of returning
false.

This patch special-cases REQUIRES_EXPR to always stream the chain of
decls in its first operand.  As a drive-by improvement we also remove a
fixme about checking uncontexted PARM_DECLs.

	PR c++/122789

gcc/cp/ChangeLog:

	* module.cc (trees_out::core_vals): Treat REQUIRES_EXPR
	specially and stream the chained decls of its first operand.
	(trees_in::core_vals): Likewise.
	(trees_out::tree_node): Check the PARM_DECLs we see are what we
	expect.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/concept-12_a.C: New test.
	* g++.dg/modules/concept-12_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>