c++: omp reduction cleanups

c++: omp reduction cleanups

omp reductions are modeled as nested functions, which is a thing C++
doesn't have.  Leading to much confusion until I figured out what was
happening.  Not helped by some duplicate code and inconsistencies in
the dependent and non-dependent paths.  This patch removes the parser
duplication and fixes up some bookkeeping.  Added some asserts and
comments too.

	gcc/cp/
	* parser.c (cp_parser_omp_declare_reduction): Refactor to avoid
	code duplication.  Update DECL_TI_TEMPLATE's context.
	* pt.c (tsubst_expr): For OMP reduction function, set context to
	global_namespace before pushing.
	(tsubst_omp_udr): Assert current_function_decl, add comment about
	decl context.