c++: Substitute into function parms in lexical order [PR96560]

c++: Substitute into function parms in lexical order [PR96560]

This makes tsubst_arg_types substitute into a function's parameter types
in left-to-right instead of right-to-left order, in accordance with DR 1227.

	DR 1227
	PR c++/96560

gcc/cp/ChangeLog:

	* pt.c (tsubst_arg_types): Rearrange so that we substitute into
	TYPE_ARG_TYPES in forward order while short circuiting
	appropriately.  Adjust formatting.

gcc/testsuite/ChangeLog:

	* g++.dg/template/sfinae-dr1227.C: New test.