SI-7974 Avoid calling nonPrivateMember after erasure

One example were this would lead to subtle bugs otherwise is
Symbol_apply, where after erasure an overloaded symbol containing
Symbol#apply and UniquenessCache#apply is returned.

    // findMember considered harmful after erasure; e.g.
    //
    // scala> exitingErasure(Symbol_apply).isOverloaded
    // res27: Boolean = true
SI-7974 Clean up and test 'Symbol-handling code in CleanUp

Looks like the transformation did never happen because the pattern
failed to match. Why did the pattern fail to match? Because the
Symbol.apply we see in the tree claims to be a method while
Symbol_apply defined in Definitions wants to be a value.

This issue was caused because nonPrivateMember starts spitting out
overloaded symbols after erasure.
This issue has been fixed in the earlier commit, so what happens in
this commit is adding tests and fixing documentation.
Merge pull request #3149 from soc/SI-7974

 Fix broken 'Symbol-handling code in CleanUp