SI-6387 Clones accessor before name expansion

When a symbol's name is expanded due to a conflict during
composition (e.g. multiple traits with same-named members, but
which are not both visible at the language level in the concrete
class) the compiler renames some symbols with expanded names which
embed the full name of the declaring class to avoid clashes.

In the rare cases when the accessor overrides the member in base
class, such expansion either results in AbstractMethodError when
the base method is abstract, or, even worse, can change the
semantics of the program.

To avoid such issues, we clone the accessor symbol, clear its
ACCESSOR flag and enter the symbol with an unchanged name.
Merge pull request #2288 from paulp/pr/2273-with-edits

SI-6387 Clones accessor before name expansion