fixes #19171; have openArray converted from ptr UncheckedArray be mutable (#23882)

Makes `toOpenArray(x: ptr UncheckedArray)` always return a `var
openArray` regardless of if `x` is mutable.

tests/openarray/tuncheckedarray.nim

@@ -0,0 +1,19 @@
+discard """
+  exitcode: 0
+  targets: "c cpp"
+"""
+
+proc main =
+  block: # issue 19171
+    var a = ['A']
+    proc mutB(x: var openArray[char]) =
+      x[0] = 'B'
+    mutB(toOpenArray(cast[ptr UncheckedArray[char]](addr a), 0, 0))
+    doAssert a[0] == 'B'
+    proc mutC(x: var openArray[char]; c: char) =
+      x[0] = c
+    let p = cast[ptr UncheckedArray[char]](addr a)
+    mutC(toOpenArray(p, 0, 0), 'C')
+    doAssert p[0] == 'C'
+
+main()