c++: partially inst requires-expr in noexcept-spec [PR101043]

c++: partially inst requires-expr in noexcept-spec [PR101043]

Here we're ICEing from strip_typedefs for the partially instantiated
requires-expression when walking its REQUIRES_EXPR_EXTRA_ARGS which
in this case is a TREE_LIST with non-empty TREE_PURPOSE (to hold the
captured local specialization 't' as per build_extra_args) which
strip_typedefs doesn't expect.

We can probably skip walking REQUIRES_EXPR_EXTRA_ARGS at all since it
shouldn't contain any typedefs in the first place, but it seems safer
and more generally useful to just teach strip_typedefs to handle non-empty
TREE_PURPOSE the obvious way.  (The code asserts TREE_PURPOSE was empty
even since since its inception i.e. r189298.)

	PR c++/101043

gcc/cp/ChangeLog:

	* tree.cc (strip_typedefs_expr) <case TREE_LIST>: Handle
	non-empty TREE_PURPOSE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires37.C: New test.