make privateAccess work with generic types and generic instantiations; fix a SIGSEGV (#18260)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

tests/stdlib/mimportutils.nim

@@ -13,5 +13,20 @@ type
     hd1: float
   PA* = ref A
   PtA* = ptr A
+  E*[T] = object
+    he1: int
+  FSub[T1, T2] = object
+    h3: T1
+    h4: T2
+  F*[T1, T2] = ref FSub[T1, T2]
+  G*[T] = ref E[T]
+  H3*[T] = object
+    h5: T
+  H2*[T] = H3[T]
+  H1*[T] = ref H2[T]
+  H*[T] = H1[T]
+
+type BAalias* = typeof(B.default)
+  # typeof is not a transparent abstraction, creates a `tyAlias`
 
 proc initB*(): B = B()