openmp: Add support for streaming metadirectives and resolving them after LTO

openmp: Add support for streaming metadirectives and resolving them after LTO

This patch adds support for streaming metadirective Gimple statements during
LTO, and adds a metadirective expansion pass that runs after LTO.  This is
required for metadirectives with selectors that can only be resolved from
within the accel compiler.

2022-01-25  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* Makefile.in (OBJS): Add omp-expand-metadirective.o.
	* gimple-streamer-in.cc (input_gimple_stmt): Add case for
	GIMPLE_OMP_METADIRECTIVE.  Handle metadirective labels.
	* gimple-streamer-out.cc (output_gimple_stmt): Likewise.
	* omp-expand-metadirective.cc: New.
	* passes.def: Add pass_omp_expand_metadirective.
	* tree-pass.h (make_pass_omp_expand_metadirective): New prototype.