[Ada] Use Standard.Natural on bit references to packed arrays

[Pierre-Marie de Rodat]

This changes the type set on bit references made to packed arrays:
instead of Universal_Integer, it is set to Standard.Natural.

The reason is that Universal_Integer must be a type as large as the
largest supported integer type and, therefore, can be much larger than
what is really needed here.

No functional changes.

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

2020-06-02  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_pakd.adb (Expand_Packed_Bit_Reference): Change type of
	reference from Universal_Integer to Standard_Natural.
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