OpenMP/Fortran: Reject declarations between target + teams

While commit r14-2754-g2e31fe431b08b0302e1fa8a1c18ee51adafd41df
detected executable statements, declarations do not show up as
executable statements.  Hence, we now check whether the first
statement after TARGET is TEAMS - such that we can detect data
statements like type or variable declarations.  Fortran semantics
ensures that only executable directives/statemens can come after
'!$omp end teams' such that those can be detected with the
previous check.

Note that statements returning ST_NONE such as 'omp nothing' or
'omp error at(compilation)' will still slip through.

	PR fortran/110725
	PR middle-end/71065

gcc/fortran/ChangeLog:

	* gfortran.h (gfc_omp_clauses): Add target_first_st_is_teams.
	* parse.cc (parse_omp_structured_block): Set it if the first
	statement in the structured block of a TARGET is TEAMS or
	a combined/composite starting with TEAMS.
	* openmp.cc (resolve_omp_target): Also show an error for
	contains_teams_construct without target_first_st_is_teams.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/teams-6.f90: New test.