[Ada] Add contracts to Strings libraries

[Pierre-Marie de Rodat]

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

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

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.
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