[Ada] Add contracts to Strings libraries

This patch adds contracts to Ada.Strings libraries, in order to remove
warnings when using these libraries in SPARK.

2019-07-10  Joffrey Huguet  <huguet@adacore.com>

gcc/ada/

	* libgnat/a-strbou.ads, libgnat/a-strfix.ads,
	libgnat/a-strunb.ads, libgnat/a-strunb__shared.ads: Add global
	contracts, contract cases, preconditions and postconditions to
	procedures and functions.

From-SVN: r273334

gcc/ada/ChangeLog

+2019-07-10  Joffrey Huguet  <huguet@adacore.com>
+
+	* libgnat/a-strbou.ads, libgnat/a-strfix.ads,
+	libgnat/a-strunb.ads, libgnat/a-strunb__shared.ads: Add global
+	contracts, contract cases, preconditions and postconditions to
+	procedures and functions.
+
 2019-07-10  Doug Rupp  <rupp@adacore.com>
 
 	* sysdep.c (__gnat_is_file_not_found_error): Reformulate to also