Fix incorrect caching with path-dependent types

The added test case used to fail Ycheck:typer with the seemingly identicals:

    Found:    (a: (aa : A{type B = Int}), b: a.B): CCPoly[(aa : A{type B = Int})]
    Required: (a: (aa : A{type B = Int}), b: a.B): CCPoly[(aa : A{type B = Int})]

In fact one of the `aa` is a a TypeVar instantiated to `A {type B = Int }`. The
MethodType comparison failed the signature check because the `a.B` where `a` is
backed by a type variable had a stale signature cached.

Fixed by changing `isProvisional` to traverse BoundTypes (`a` is a
`TermParamRef` here).

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -144,6 +144,9 @@ object Types extends TypeUtils {
               !t.isPermanentlyInstantiated || test(t.permanentInst, theAcc)
             case t: LazyRef =>
               !t.completed || test(t.ref, theAcc)
+            case t: BoundType =>
+              (t: Type).mightBeProvisional = false // break cycles
+              test(t.underlying, theAcc)
             case _ =>
               (if theAcc != null then theAcc else ProAcc()).foldOver(false, t)
         end if

tests/pos/dep-poly-class.scala

@@ -0,0 +1,9 @@
+trait A:
+  type B
+
+class CCPoly[T <: A](a: T, b: a.B)
+
+object Test:
+  def test(): Unit =
+    val aa: A { type B = Int } = new A { type B = Int }
+    val x: CCPoly[aa.type] = CCPoly(aa, 1)