c++: Add some asserts in mangle.c

c++: Add some asserts in mangle.c

The added asserts are places I noticed that we aren't adding the "on" to
distinguish an operator name from an expression involving the operator.
Hopefully that's because operator names never get there; if not, crash bug
reports will let us fix the issue.

gcc/cp/ChangeLog:

	* mangle.c (write_member_name): Add assert.
	(write_expression): Add asserts.