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.