[Ada] Refine conditions for calling Copy_Bitfield

[Pierre-Marie de Rodat]

Avoid calling Copy_Bitfield if there are volatile or independent
components that might be read or written. The test is conservative.

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

2019-09-17  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_ch5.adb (Expand_Assign_Array_Loop_Or_Bitfield): Add tests
	for potential volatile or independent components.
	* libgnat/s-bituti.adb (Copy_Small_Bitfield,
	Copy_Large_Bitfield): Move declarations to more appropriate
	place.
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