Fix regression with -Wexternal-argument-mismatch.

The attached patch fixes an ICE regresseion where undo state was not
handled properly when generating formal from actual arguments, which
occurred under certain conditions with the newly introduced
-Wexternal-argument-mismatch option.

The fix is simple: When we are generating these symbols, we no
longer need to undo anything, so we can just remove them.
I had considered adding an extra optional argument, but decided
against it on code clarity grounds.

While looking at the code, I also saw that a member of gfc_symbol
introduced with my patch should be a bitfield of width 1.

gcc/fortran/ChangeLog:

	PR fortran/119157
	* gfortran.h (gfc_symbol): Make ext_dummy_arglist_mismatch a
	one-bit bitfield
	(gfc_pop_undo_symbol): Declare prototype.
	* symbol.cc (gfc_pop_undo_symbol): New function.
	* interface.cc (gfc_get_formal_from_actual_arglist): Call it
	for artificially introduced formal variables.

gcc/testsuite/ChangeLog:

	PR fortran/119157
	* gfortran.dg/interface_57.f90: New test.

gcc/fortran/gfortran.h

@@ -2026,7 +2026,7 @@ typedef struct gfc_symbol
   /* Set if an external dummy argument is called with different argument lists.
      This is legal in Fortran, but can cause problems with autogenerated
      C prototypes for C23.  */
-  unsigned ext_dummy_arglist_mismatch;
+  unsigned ext_dummy_arglist_mismatch:1;
 
   /* Reference counter, used for memory management.
 
@@ -3736,6 +3736,7 @@ void gfc_traverse_user_op (gfc_namespace *, void (*)(gfc_user_op *));
 void gfc_save_all (gfc_namespace *);
 
 void gfc_enforce_clean_symbol_state (void);
+void gfc_pop_undo_symbol (void);
 
 gfc_gsymbol *gfc_get_gsymbol (const char *, bool bind_c);
 gfc_gsymbol *gfc_find_gsymbol (gfc_gsymbol *, const char *);

gcc/fortran/interface.cc

@@ -5836,6 +5836,8 @@ gfc_get_formal_from_actual_arglist (gfc_symbol *sym,
 	{
 	  snprintf (name, GFC_MAX_SYMBOL_LEN, "_formal_%d", var_num ++);
 	  gfc_get_symbol (name, gfc_current_ns, &s);
+	  /* We do not need this in an undo table.  */
+	  gfc_pop_undo_symbol();
 	  if (a->expr->ts.type == BT_PROCEDURE)
 	    {
 	      gfc_symbol *asym = a->expr->symtree->n.sym;

gcc/fortran/symbol.cc

@@ -3898,6 +3898,11 @@ enforce_single_undo_checkpoint (void)
   gcc_checking_assert (single_undo_checkpoint_p ());
 }
 
+void
+gfc_pop_undo_symbol ()
+{
+  latest_undo_chgset->syms.pop();
+}
 
 /* Undoes all the changes made to symbols in the current statement.  */
 

gcc/testsuite/gfortran.dg/interface_57.f90

@@ -0,0 +1,14 @@
+! { dg-do compile }
+! { dg-options "-Wexternal-argument-mismatch" }
+! PR 119157 - this used to ICE because undo state was not
+! correctly handled.
+
+MODULE lmdif_module
+  implicit none
+   CONTAINS
+      SUBROUTINE lmdif (fcn, m)
+        EXTERNAL fcn
+        integer m
+        call fcn (m)
+      END SUBROUTINE lmdif
+END MODULE