openmp: Implement discovery of implicit declare target to clauses

openmp: Implement discovery of implicit declare target to clauses

This attempts to implement what the OpenMP 5.0 spec in declare target section
says as ammended by the 5.1 changes so far (related to device_type(host)), except
that it doesn't have the device(ancestor: ...) handling yet because we do not
support it yet, and I've left so far out the except lambda note, because I need
that clarified.

2020-05-12  Jakub Jelinek  <jakub@redhat.com>

	* omp-offload.h (omp_discover_implicit_declare_target): Declare.
	* omp-offload.c: Include context.h.
	(omp_declare_target_fn_p, omp_declare_target_var_p,
	omp_discover_declare_target_fn_r, omp_discover_declare_target_var_r,
	omp_discover_implicit_declare_target): New functions.
	* cgraphunit.c (analyze_functions): Call
	omp_discover_implicit_declare_target.

	* testsuite/libgomp.c/target-39.c: New test.