[Ada] Fix wrong value of 'Size for slices of bit-packed arrays (2)

[Pierre-Marie de Rodat]

This completes the previous fix, which is not sufficient when the Size
attribute is applied to a formal object of mode In Out and the generic
is instantiated on a slice of a bit-packed array whose size is not a
multiple of the storage unit.

The hurdle here is that introducing a temporary may change the value
of the Size attribute, because a temporary is a stand-alone object and
the size of stand-alone objects is always a multiple of the storage
unit with GNAT.

Instead of trying to look through them in Expand_Size_Attribute, this
changes the expansion done by Expand_N_Slice to avoid creating these
problematic temporaries for slices of bit-packed arrays that are the
prefix of the Size attribute.

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

2019-12-12  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_attr.adb (Expand_Size_Attribute): Look directly at the
	prefix to detect the bit-packed slices.  Apply the checks last
	in case the attribute needs to be processed by the back-end.
	* exp_ch4.adb (Expand_N_Slice): Do not create a temporary for
	a prefix of the Size attribute.
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