c++: requires-expr in pack expansion using pack [PR103105]

c++: requires-expr in pack expansion using pack [PR103105]

Here after dependent substitution of {Ts...} into the alias 'wrap',
since we never partially instantiate a requires-expr, we end up with a
requires-expr whose REQUIRES_EXPR_EXTRA_ARGS contains an
ARGUMENT_PACK_SELECT (which just resolves to the parameter pack Ts).
Then when hashing the resulting dependent specialization of A, we crash
from iterative_hash_template_arg since it deliberately doesn't handle
ARGUMENT_PACK_SELECT.

Like in r12-7102-gdb5f1c17031ad8, it seems the right fix here is to
resolve ARGUMENT_PACK_SELECT arguments before storing them into an
extra args tree (such as REQUIRES_EXPR).

	PR c++/103105

gcc/cp/ChangeLog:

	* pt.cc (build_extra_args): Call preserve_args.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires29.C: New test.
	* g++.dg/cpp2a/concepts-requires29a.C: New test.