c++: P2360R0: Extend init-stmt to allow alias-decl [PR102617]

c++: P2360R0: Extend init-stmt to allow alias-decl [PR102617]

The following patch implements C++23 P2360R0.  This proposal merely
extends init-statement to contain alias-declaration.  init-statement
is used in if/for/switch.  It also removes the unsightly duplication
of code by calling cp_parser_init_statement twice.

	PR c++/102617

gcc/cp/ChangeLog:

	* parser.c (cp_parser_for): Maybe call cp_parser_init_statement
	twice.  Warn about range-based for loops with initializer here.
	(cp_parser_init_statement): Don't duplicate code.  Allow
	alias-declaration in init-statement.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/init-stmt1.C: New test.
	* g++.dg/cpp23/init-stmt2.C: New test.