[class.access.base] No cv-qualification for access to inherited members

[frederick-vs-ja]

Fixes #4952
The pointed-to naming class type should be identically cv-qualified.
The class type of the left operand and the naming class of the member should be considered cv-unqualified for the purpose of accessibility.

[languagelawyer]

@jensmaurer @zygoloid @opensdh
Maybe just remove the paragraph? It seems to say the same thing as http://eel.is/c++draft/expr.prim.id.general#3.1 (which also needs a fix, at least s/can/shall)

[opensdh]

@languagelawyer: Even aside from the "member's class" issue, that generic restriction doesn't address the possibility of rejecting the access for access control or ambiguous inheritance reasons. Perhaps there is already yet another place that checks that, though.

[frederick-vs-ja]

@jensmaurer I'm trying to use the "less clunky approach" now (with squashing and rebasing).