[Ada] Ada2020: AI12-0368 Declare expressions can be static

[Pierre-Marie de Rodat]

This patch implements AI12-0368, which allows declare expressions
to be static.

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

gcc/ada/

	* sem_res.adb (Resolve_Expression_With_Actions): Check the rules
	of AI12-0368, and mark the declare expression as static or known
	at compile time as appropriate.
	* sem_ch4.adb: Minor reformatting.
	* libgnat/a-stoufo.ads, libgnat/a-stoufo.adb: Allow up to 9
	replacement parameters. I'm planning to use this in the test
	case for this ticket.
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