fix string literal assignment with different lengths on ARC (#24083)

fixes #24080

tests/arc/tstringliteral.nim

@@ -0,0 +1,17 @@
+discard """
+  matrix: "--mm:arc; --mm:orc"
+"""
+
+block: # issue #24080
+  var a = (s: "a")
+  var b = "a"
+  a.s.setLen 0
+  b = a.s
+  doAssert b == ""
+
+block: # issue #24080, longer string
+  var a = (s: "abc")
+  var b = "abc"
+  a.s.setLen 2
+  b = a.s
+  doAssert b == "ab"