Separate OpenACC kernels regions in data and parallel parts

Separate OpenACC kernels regions in data and parallel parts

This is the first in a series of patches that completely rework the handling
of the OpenACC "kernels" directive. In the future, kernels regions will be
transformed into data regions containing a sequence of serial and parallel
offloaded regions. This first patch sets up a new pass that is responsible
for this transformation, and in a first step constructs the new data region
containing a parallel region with the original kernels region's body.

	gcc/
	* Makefile.in: Add...
	* omp-oacc-kernels.c: ... this new file for the kernels conversion
	pass.
	* flag-types.h (enum openacc_kernels): Add "split" style.  Adjust
	all users.
	* doc/invoke.texi (-fopenacc-kernels): Update.
	* passes.def: Add pass_convert_oacc_kernels to pipeline.
	* tree-pass.h (make_pass_convert_oacc_kernels): Add declaration.
	gcc/testsuite/
	* c-c++-common/goacc/kernels-conversion.c: New test.
	* gfortran.dg/goacc/kernels-conversion.f95: Likewise.
	* c-c++-common/goacc/if-clause-2.c: Update.
	* gfortran.dg/goacc/kernels-tree.f95: Likewise.

(cherry picked from openacc-gcc-9-branch commit
f769c55eb290873a16b3da32fa15c03ff0d5578e)