[Ada] Add "Global => null" contracts to Ada.Calendar routines

[Ada] Add "Global => null" contracts to Ada.Calendar routines

Routines in Ada.Real_Time are already annotated with Global => null
contracts to suppress spurious warnings from the flow analysis in
GNATprove. This patch adds such contracts to Ada.Calendar. No change in
runtime behavior expected.

2018-12-11  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* libgnat/a-calend.ads: Add "Global => null" contracts to pure
	routines.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@267018 138bc75d-0d04-0410-961f-82ee72b054a4