backport: re PR c++/60228 (ICE using lambda in #pragma omp declare reduction)

backport: re PR c++/60228 (ICE using lambda in #pragma omp declare reduction)

	Backported from mainline
	2019-11-29  Jakub Jelinek  <jakub@redhat.com>

	PR c++/60228
	* parser.c (cp_parser_omp_declare_reduction_exprs): If
	processing_template_decl, wrap the combiner or initializer
	into EXPR_STMT.
	* decl.c (start_preparsed_function): Don't start a lambda scope
	for DECL_OMP_DECLARE_REDUCTION_P functions.
	(finish_function): Don't finish a lambda scope for
	DECL_OMP_DECLARE_REDUCTION_P functions, nor cp_fold_function
	them nor cp_genericize them.
	* mangle.c (decl_mangling_context): Look through
	DECL_OMP_DECLARE_REDUCTION_P functions.
	* semantics.c (expand_or_defer_fn_1): For DECL_OMP_DECLARE_REDUCTION_P
	functions, use tentative linkage, don't keep their bodies with
	-fkeep-inline-functions and return false at the end.

	* g++.dg/gomp/openmp-simd-2.C: Don't expect bodies for
	DECL_OMP_DECLARE_REDUCTION_P functions.

	* testsuite/libgomp.c++/udr-20.C: New test.
	* testsuite/libgomp.c++/udr-21.C: New test.