string to string conversion keeps the dest type (#9323)

Fixes #9322

tests/distinct/t9322.nim

@@ -0,0 +1,12 @@
+discard """
+  output: apr
+"""
+
+type Fix = distinct string
+
+proc `$`(f: Fix): string {.borrow.}
+
+proc mystr(s: string) =
+  echo s
+
+mystr($Fix("apr"))