c++: mangling tweaks

c++: mangling tweaks

Most of this is introducing the abi_check function to reduce the verbosity
of most places that check -fabi-version.

The start_mangling change is to avoid needing to zero-initialize additional
members of the mangling globals, though I'm not actually adding any.

The comment documents existing semantics.

gcc/cp/ChangeLog:

	* mangle.cc (abi_check): New.
	(write_prefix, write_unqualified_name, write_discriminator)
	(write_type, write_member_name, write_expression)
	(write_template_arg, write_template_param): Use it.
	(start_mangling): Assign from {}.
	* cp-tree.h: Update comment.