fixes #20422; emit nimPrepareStrMutationV2 for toOpenArray to keep th… (#21459)

fixes #20422; emit nimPrepareStrMutationV2 for toOpenArray to keep the abstraction of mutable strings which have immutable string literals

tests/arc/topenarray.nim

@@ -6,6 +6,8 @@ Nim
 '''
   matrix: "--gc:arc -d:useMalloc; --gc:arc"
 """
+{.experimental: "views".}
+
 block: # bug 18627
   proc setPosition(params: openArray[string]) =
     for i in params.toOpenArray(0, params.len - 1):
@@ -38,3 +40,13 @@ block: # bug #20954
       var v: seq[int]
 
       echo len(toOpenArray(v, 20, 30))
+
+# bug #20422
+
+proc f(a: var string) =
+  var v = a.toOpenArray(1, 3)
+  v[0] = 'a'
+
+var a = "Hello"
+f(a)
+doAssert a == "Hallo"