[Ada] Ada2020: Reduction expressions

[Ada] Ada2020: Reduction expressions

This patch dismantles the prototype implementation of the first proposal
for Reduction expressions, one of the important potentially parallel
constructs for Ada2020. The ARG is going in a different direction with
a simpler syntax.

2018-05-22  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_ch4.ads, exp_ch4.adb, exp_util.adb, expander.adb: Remove mention
	of N_Reduction_Expression and N_Reduction_Expression_Parameter.
	* par-ch4.adb: Remove parsing routines for reduction expressions.
	* sem.adb, sinfo.ads, sinfo.adb, sem_ch4.ads, sem_ch4.adb, sem_res.adb,
	sem_spark.adb, sprint.adb: Remove analysis routines for reduction
	expressions.

From-SVN: r260524