c++: parameterized requires-expr as default argument [PR101725]

c++: parameterized requires-expr as default argument [PR101725]

Here we're rejecting the default template argument

  requires (T t) { x(t); }

because we consider the 't' in the requirement to be a local variable
(according to local_variable_p), and we generally forbid local variables
from appearing inside default arguments.  We can perhaps fix this by
giving special treatment to parameters introduced by requires-expressions,
but DR 2082 relaxed the restriction about local variables appearing within
default arguments to permit them inside unevaluated operands thereof.
So this patch just implements DR 2082 which also fixes this PR since a
requires-expression is an unevaluated context.

	PR c++/101725
	DR 2082

gcc/cp/ChangeLog:

	* cp-tree.h (unevaluated_p): Return true for REQUIRES_EXPR.
	* decl.c (local_variable_p_walkfn): Don't walk into unevaluated
	operands.
	* parser.c (cp_parser_primary_expression) <case CPP_NAME>: Never
	reject uses of local variables in unevaluated contexts.
	* tree.c (cp_walk_subtrees) <case REQUIRES_EXPR>: Increment
	cp_unevaluated_operand.  Use cp_walk_tree directly instead of
	WALK_SUBTREE to avoid the goto.  Use REQUIRES_EXPR_REQS instead
	of TREE_OPERAND directly.

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2082.C: New test.
	* g++.dg/cpp2a/concepts-uneval4.C: New test.

(cherry picked from commit 9707d2e5dbb92d2bc990c922461a5a16ae652319)