[Ada] Unnesting: do not eliminate subprogram whose address is taken

[Ada] Unnesting: do not eliminate subprogram whose address is taken

2018-12-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_unst.adb (Register_Subprogram): A subprogram whose address
	is taken (through attribute Address or Access) is reachable and
	cannot be eliminated, even if there is no explicit call to it:
	the address may be used in an indirect call or in some
	address-related operation.

From-SVN: r267014