Fix PR70182 -- missing "on" in mangling of unresolved operators

The ABI says:

<unresolved-name>
   ::= [gs] <base-unresolved-name>
   ::= sr <unresolved-type> <base-unresolved-name>
   ::= srN <unresolved-type> <unresolved-qualifier-level>+ E <base-unresolved-name>
   ::= [gs] sr <unresolved-qualifier-level>+ E <base-unresolved-name>

<base-unresolved-name>
   ::= <simple-id>
   ::= on <operator-name>
   ::= on <operator-name> <template-args>
   ::= dn <destructor-name

libiberty:

	PR c++/70182
	* cp-demangle.c (d_unqualified_name): Handle "on" for
	operator names.
	* testsuite/demangle-expected: Add tests.

gcc/cp:

	PR c++/70182
	* mangle.c (write_template_args): Add "on" for operator names.

gcc:

	PR c++/70182
	* doc/invoke.texi (fabi-version): Mention mangling fix for
	operator names.

From-SVN: r244567