[RFA] Extend demangler to recognize destructors/constructors

[Michael Elizabeth Chastain]

I like the enums.  But I would like to have an explicit enum for
gnu_v3_not_a_ctor and gnu_v3_not_a_dtor, rather than using a literal
value of 0 which is not in the enum (consider what gdb will show people).

Also in the peek_char / next_char changes, you changed where
the current char is during the call to result_add_string.  I don't
know if that's important.

Michael