fix compiler crash with uncheckedAssign and range/distinct discrims [backport] (#25585)

On simple code like:

```nim
type Foo = object
  case x: range[0..7]
  of 0..2:
    a: string
  else:
    b: string

var foo = Foo()
{.cast(uncheckedAssign).}:
  foo.x = 5
```

The compiler tries to generate a destructor for the variant fields by
checking if the discrim is equal to the old one, but the type is not
skipped when looking for an `==` operator in system, so any
discriminator with type `range`/`distinct`/etc crashes with:

```
(10, 9) Error: can't find magic equals operator for type kind tyRange
```

This is fixed by just skipping abstract types.

(cherry picked from commit 7a87e7d199)

tests/arc/tcaseobj.nim

@@ -365,3 +365,28 @@ proc do2(x: int, e: ItemExt): seq[(string, ItemExt)] =
   do1(x).map(proc(v: (string, Item)): auto = (v[0], ItemExt(a: v[1], b: e.b)))
 
 doAssert $do2(0, ItemExt(a: Item(kind: 1, c: "second"), b: "third")) == """@[("zero", (a: (kind: 1, c: "first"), b: "third"))]"""
+
+block:
+  type RangeCrash = object
+    case x: range[0..7]
+    of 0..2:
+      a: string
+    else:
+      b: string
+
+  var rangeCrash = RangeCrash()
+  {.cast(uncheckedAssign).}:
+    rangeCrash.x = 5
+
+block:
+  type Discrim = distinct uint8
+  type DistinctCrash = object
+    case x: Discrim
+    of Discrim(0)..Discrim(2):
+      a: string
+    else:
+      b: string
+
+  var distinctCrash = DistinctCrash()
+  {.cast(uncheckedAssign).}:
+    distinctCrash.x = Discrim(5)