retain forced open undeclared ident information (#22019)

tests/lookups/tundeclaredmixin.nim

@@ -0,0 +1,17 @@
+discard """
+  nimout: '''
+  mixin nothing, add
+'''
+"""
+
+# issue #22012
+
+import macros
+
+expandMacros:
+  proc foo[T](): int =
+    # `nothing` is undeclared, `add` is declared
+    mixin nothing, add
+    123
+
+  doAssert foo[int]() == 123