ada: Compiler crash on early alignment clause

ada: Compiler crash on early alignment clause

This patch fixes a bug: if "for T'Alignment use..." is followed
by "for T use (<enum rep>);" the compiler crashes. A workaround is
to move the alignment clause after the enumeration rep clause.

gcc/ada/

	* sem_ch13.ads (Set_Enum_Esize): Do not set alignment.
	* sem_ch13.adb (Set_Enum_Esize): Do not set alignment. Archaeology
	seems to show that this line of code dates from when "Alignment =
	0" meant "the Alignment is not known at compile time" and "the
	Alignment is not yet known at compile time" as well as "the
	Alignment is zero". In any case, it seems to be unnecessary, and
	in this case harmful, because gigi would crash. Alignment_Clause
	is set (because there is one), so gigi would query the Alignment,
	but Alignment was destroyed.