Fix regression with -Wexternal-argument-mismatch.

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.