unexpandedName needs hardening.

[odersky]

Unexpanded name currently fails for names that have two consecutive '$'s in them. For instance, the super accessor of += in trait T would be encoded as "T$$super$$plus$eq". Its unexpandedName
will then be "plus$eq", where "super$$plus$eq" would have been correct.

[smarter]

Somewhat related to #30.

[smarter]

Hopefully fixed by the semantic name PR: #2128