[extension types] Member conflicts ignore precluded members (#3486)

Adjust the rules about extension type member conflicts: precluded members do not create conflicts.

This PR changes the rule about member conflicts such that a member which is precluded does not conflict with any other members. It is an adjustment of the rules added by #3470.

For example, if an extension type `E1` declares a method named `m` and an extension type `E2` declares a setter named `m=` and has `implements E1`, then `E2` does not "inherit" the method named `m` (it is precluded because it would conflict with the setter).

In this PR we add an extra rule which says that it is not a conflict if `E2` has two superinterfaces `E1a` and `E1b`, and `E1a` declares a method named `m` and `E1b` declares a getter named `m`, and `E2` again declares a setter named `m=` and has `implements E1a, E1b`. The point as that `E2` will "inherit" the getter (so it has a full setter/getter pair), but not the method (because it is precluded by the setter), and (here comes the new thing) the method is ignored during conflict checks because it is precluded, so we avoid the useless error about `m` being ambiguous that we'd otherwise have because of `implements E1a, E1b`.

accepted/future-releases/extension-types/feature-specification.md

@@ -906,8 +906,9 @@ On the other hand, it can occur in other ways, e.g., as a type argument of
 a superinterface of a class.*
 
 It is a compile-time error if _DV_ is an extension type declaration, and
-_DV_ has a non-extension type member named `m` as well as an extension type
-member named `m`, for any `m`. *In case of conflicts, _DV_ must declare a
+_DV_ has a non-extension type member named `m` which is not precluded by
+_DV_ as well as an extension type member named `m` which is not precluded
+by _DV_, for any `m`. *In case of conflicts, _DV_ must declare a
 member named `m` to resolve the conflict.*
 
 It is a compile-time error if _DV_ is an extension type declaration, and
@@ -918,8 +919,8 @@ conflict.*
 
 A compile-time error occurs if an extension type declaration _DV_ has
 two extension type superinterfaces `V1` and `V2`, and both `V1` and `V2`
-has an extension type member named `m`, and the two members have distinct
-declarations.
+has an extension type member named `m` that is not precluded by _DV_, and
+the two members have distinct declarations.
 
 *In other words, an extension type member conflict is always an error, even
 in the case where they agree perfectly on the types. _DV_ must override the