Refine how we check for stability

In particular, must continue to use the type's prefix, rather than the
denotation's prefix.  That way, e.g. in tests/neg/i3812b.scala,
referencing value class'\''s value, will fail on the non-stable value
class prefix, rather than the denotation'\''s prefix, which is NoSymbol.

[Cherry-picked e6012a0]

Author: dwijnand

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -669,7 +669,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
        && !tree.isType
        && !pt.isInstanceOf[ApplyingProto]
        && !tree.tpe.match
-        case tp: NamedType => tp.denot.hasAltWith(_.symbol.namedType.isStable)
+        case tp: NamedType => tp.denot.hasAltWith(_.symbol.isStableMember && tp.prefix.isStable || tp.info.isStable)
         case tp            => tp.isStable
        && !isWildcardArg(tree)
     then