[Ada] Move SPARK borrow-checker to gnat2why codebase

[Pierre-Marie de Rodat]

Unit sem_spark was implementing the borrow-checker for the support of
ownership pointers in SPARK. It has been moved to gnat2why codebase to
facilitate its evolution and allow the more powerful flow analysis to
provide its results for better analysis on pointers.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-09-19  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* gcc-interface/Make-lang.in: Remove references to sem_spark.
	* sem_spark.adb, sem_spark.ads: Remove unit.
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