Test global identifiers against what is specified interfaces.

Test global identifiers against what is specified interfaces.

Apart from calling gfc_compare_interfaces to check interfaces against
global identifiers, this also sets and check a few sym->error flags
to avoid duplicate error messages.  I thought about issuing errors
on mismatched interfaces, but when the procedure is not invoked,
a warning should be enough to alert the user.

gcc/fortran/ChangeLog:

	PR fortran/27318
	* frontend-passes.c (check_against_globals): New function.
	(gfc_check_externals): Split; also invoke check_against_globals
	via gfc_traverse_ns.
	(gfc_check_externals0): Recursive part formerly in
	gfc_check_externals.
	* resolve.c (resolve_global_procedure): Set sym->error on
	interface mismatch.
	* symbol.c (ambiguous_symbol): Check for, and set sym->error.

gcc/testsuite/ChangeLog:

	PR fortran/27318
	* gfortran.dg/error_recovery_1.f90: Adjust test case.
	* gfortran.dg/use_15.f90: Likewise.
	* gfortran.dg/interface_47.f90: New test.