libexpr: Deprecate or in non-keyword positions

Test cases courtesy of rhenrdic

Co-authored-by: Commentator2.0 <lix@crystal-cavern.systems>
Change-Id: Id8df684ddd00d07b49e1c9e68bf41ee6c0e6887c

tests/nixpkgs/eval.nix

@@ -15,6 +15,7 @@ let
     "broken-string-escape"
     "rec-set-merges"
     "rec-set-dynamic-attrs"
+    "or-as-identifier"
   ];
 in
 

tests/nixpkgs/lib.nix

@@ -24,6 +24,7 @@ let
     "broken-string-escape"
     "rec-set-merges"
     "rec-set-dynamic-attrs"
+    "or-as-identifier"
   ];
 
   env.NIX_CONFIG = "extra-deprecated-features = ${concatStringsSep " " deprecatedFeatures}";