Fortran] PR93309 – permit repeated 'implicit none(external)'

	Backported from mainline
	2020-01-21  Tobias Burnus  <tobias@codesourcery.com>

	PR fortran/93309
	* interface.c (gfc_procedure_use): Also check parent namespace for
	'implict none (external)'.
	* symbol.c (gfc_get_namespace): Don't set has_implicit_none_export
	to parent namespace's setting.

	Backported from mainline
	2020-01-21  Tobias Burnus  <tobias@codesourcery.com>

	PR fortran/93309
	* gfortran.dg/external_implicit_none_2.f90: New.