Revert "Drop redundant butNot = Param clause in isAnchor"

[EugeneFlesselle]

This reverts commit 9d88c80.
Closes #21521

The ClassTypeParamCreationFlags include both TypeParam and Deferred.
In effect, a class type parameter was considered an anchor for implicit search, by sym.is(Deferred) as a sufficient condition.

For a failing example, one can try asserting:

|| sym.is(Deferred).ensuring(_ == sym.is(Deferred, butNot = Param))

in ImplicitRunInfo#isAnchor and a test with summon[Ordering[Int]].

In that example, at least, the flags happen to be set by Scala2Unpickler#readDisambiguatedSymbol:

scala3/compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

Line 560 in 614170f

if (flags.is(TypeParam)) flags1 |= owner.typeParamCreationFlags

[som-snytt]

Without knowing anything about it, removing Deferred from the flag set doesn't break anything:

   val ClassTypeParamCreationFlags: FlagSet =
-    TypeParam | Deferred | Private | Local
+    TypeParam | /*Deferred |*/ Private | Local

Not sure what the minimum test is for the new issue.

[EugeneFlesselle]

Without knowing anything about it, removing Deferred from the flag set doesn't break anything:

Indeed, I'm not sure about it myself either, but we may want to drop the Deferred flag from there.
Still, this is not the only source of such a composition of flags, the assertion in isAnchor continues failing after having updated the ClassTypeParamCreationFlags.