Avoid introducing 'create' mapping clauses for loop index variables in kernels regions

Avoid introducing 'create' mapping clauses for loop index variables in kernels regions

	gcc/
	* omp-oacc-kernels.c (find_omp_for_index_vars_1,
	find_omp_for_index_vars): New functions.
	(maybe_build_inner_data_region): Add IDX_VARS argument. Don't add
	CREATE mapping clauses for loop index variables.  Set TREE_ADDRESSABLE
	flag on newly-mapped declarations as a side effect.
	(decompose_kernels_region_body): Call find_omp_for_index_vars.  Don't
	create PRESENT clause for loop index variables.  Pass index variable
	set to maybe_build_inner_data_region.

(cherry picked from openacc-gcc-9-branch commit
2987e142892f89f0e8fd3a514681d5903c394beb)