[Ada] Fix on computation of packed array size in case of error

[Pierre-Marie de Rodat]

In case of compilation error, the low and high bounds of the array
type might have been replaced by an error node. Deal with this case
by checking that the bounds are known at compile time.

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

gcc/ada/

	* layout.adb (Layout_Type): Add guard before calling Expr_Value.
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