The attached patch fixes PR fortran/91066.  The original 
code was feeding a nonsense string of tokens to the
assembler causing it to toss its cookies.  It turns out
that gfortran was not enforcing the constraint C1547
from Fortran 2018.  The attached patch now performs 
that check.  Regression tested on x86_64-*-freebsd.
OK to commit?

2019-04-19  Steven G. Kargl  <kargl@gcc.gnu.org>

	PR fortran/90166
	* decl.c (in_module_or_interface): New function to check that the
	current state is in a module, submodule, or interface.
	(gfc_match_prefix): Use it.

	PR fortran/90166
	* gfortran.dg/submodule_22.f08: Add additional dg-error comments.

-- 
Steve