vec: Simplify use with C++11 range-based 'for'.

vec: Simplify use with C++11 range-based 'for'.

It looks cleaner if we can use a vec* directly as a range for the C++11
range-based 'for' loop, without needing to indirect from it, and also works
with null pointers.

The change in cp_parser_late_parsing_default_args is an example of how this
can be used to simplify a simple loop over a vector.  Reverse or subset
iteration will require adding range adaptors.

I deliberately didn't format the new overloads for etags since they are
trivial.

gcc/ChangeLog:

	* vec.h (begin, end): Add overloads for vec*.
	* tree.c (build_constructor_from_vec): Remove *.

gcc/cp/ChangeLog:

	* decl2.c (clear_consteval_vfns): Remove *.
	* pt.c (do_auto_deduction): Remove *.
	* parser.c (cp_parser_late_parsing_default_args): Change loop
	to use range 'for'.