[Ada] Disable wrong computation of offsets within multidimensional arrays

[Pierre-Marie de Rodat]

Routine Indexed_Component_Bit_Offset is meant to return the first bit
position of an array component, but it only examined the first index
expression and necessarily produced wrong results for multidimensional
arrays.

Since this routine is only used for warnings, it is safe to simply
disable this wrong code and behave just like if the offsets within
a multidimensional array would not be known at compile time.

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

gcc/ada/

	* sem_util.adb (Indexed_Component_Bit_Offset): Return an unknown
	offset for components within multidimensional arrays; remove
	redundant parens.
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