expander: fix local-expand and dropped macro definition

If `local-expand` with a 'module-begin context introduces a macro
definition, but the definition is dropped while a non-macro definition
is later introduced, then make sure references go to the non-macro
definition.

This change also addresses a related scenario, where a
`local-expand`-discovered macro definition is not dropped, but it is
given an extra scope --- which amounts to the same thing from the
expander's perspective.

pkgs/racket-test-core/tests/racket/module.rktl

@@ -2620,6 +2620,40 @@ case of module-leve bindings; it doesn't cover local bindings.
        (require (submod "." m "..")))))
  exn:fail?)
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; If `local-expand` creates a macro binding, and if
+;; the definition is discarded with a non-macro definition
+;; is introduced, then make sure the non-macro definition
+;; is referenced.
+
+(module discards-module-begin-macro-definition racket/base
+
+  (module mb racket/base
+    (require (for-syntax racket/base))
+    (provide (except-out (all-from-out racket/base)
+                         #%module-begin)
+             (rename-out [module-begin #%module-begin]))
+    (define-syntax (module-begin stx)
+      (syntax-case stx ()
+        [(_ a b)
+         (begin
+           (local-expand #'(#%module-begin a) 'module-begin null)
+           #'(#%module-begin b))])))
+
+  (module use (submod ".." mb)
+    (begin
+      (require (for-syntax racket/base))
+      (define-syntax (m stx) #''not-ok))
+    (begin
+      (define (m) 'ok)
+      (define result (m))
+      (provide result)))
+
+  (require 'use)
+  (provide result))
+
+(test 'ok dynamic-require ''discards-module-begin-macro-definition 'result)
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/expander/expand/module.rkt

@@ -777,6 +777,9 @@
                                                       #:frame-id frame-id
                                                       #:requires+provides requires+provides
                                                       #:in exp-body))
+          (for ([sym (in-list syms)])
+            ;; In case `local-expand` created a binding with `sym` to a transformer
+            (namespace-unset-transformer! m-ns phase sym))
           (add-defined-syms! requires+provides syms phase)
           (log-expand partial-body-ctx 'exit-prim
                       (datum->syntax #f `(,(m 'define-values) ,ids ,(m 'rhs)) exp-body))