[Ada] Crash on right shift operator for signed integers

[Pierre-Marie de Rodat]

This patch fixes an error in the compiler whereby right shift operators
on signed integers cause a compile-time crash when said shift operators
are provided via pragma Provide_Shift_Operators.

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

gcc/ada/

	* doc/gnat_rm/intrinsic_subprograms.rst (Shifts and Rotates):
	Document behavior on negative numbers
	* gnat_rm.texi: Regenerate.
	* sem_eval.adb (Fold_Shift): Set modulus to be based on the RM
	size for non-modular integer types.
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