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

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

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.

From-SVN: r279293