Do not walk into type sub-nodes for cast/conv expr (#10616)

tests/destructor/tcast.nim

@@ -0,0 +1,18 @@
+discard """
+  cmd: '''nim c --newruntime $file'''
+"""
+
+# Make sure we don't walk cast[T] type section while injecting sinks/destructors
+block:
+  type
+    XY[T] = object
+      discard
+
+  proc `=`[T](x: var XY[T]; v: XY[T]) {.error.}
+  proc `=sink`[T](x: var XY[T]; v: XY[T]) {.error.}
+
+  proc main[T]() =
+    var m = cast[ptr XY[T]](alloc0(sizeof(XY[T])))
+    doAssert(m != nil)
+
+  main[int]()