Add toOpenArray[T](ptr UncheckedArray[T]) for clarity. (#9316)

* Add `toOpenArray[T](ptr UncheckedArray[T])` for clarity.  `ptr array[0,T]`
for some unchecked type already works but A) `UncheckedArray` seems to be
the intended future way for this kind of access, and B) essentially all use
cases will have a `ptr` for that kind of array source and this call signature
lets callers drop the trailing `[]` corresponding to that `ptr` deref.
This PR relates to issue https://github.com/nim-lang/Nim/issues/9001 .

* Add a test for toOpenArray() for UncheckedArray[T]s.

tests/system/tsystem_misc.nim

@@ -15,6 +15,7 @@ discard """
 1
 2
 3
+2
 48
 49
 50
@@ -98,6 +99,10 @@ doAssertRaises(IndexError):
 doAssertRaises(IndexError):
   foo(toOpenArray(arrNeg, -1, -3))
 
+type seqqType = ptr UncheckedArray[int]
+let qData = cast[seqqType](addr seqq[0])
+oaFirstElm(toOpenArray(qData, 1, 3))
+
 proc foo(a: openArray[byte]) =
   for x in a: echo x