[Ada] Add pragma Annotate for CodePeer analysis

[Pierre-Marie de Rodat]

CodePeer issues a spurious message on the analysis of the loop variant
in s-widthi.adb. Similarly to the same for s-widthu.adb, add a pragma
Annotate to justify this message.

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

gcc/ada/

	* libgnat/s-widthi.adb: Add pragma Annotate.
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