[Ada] Spurious error on synchronous refinement

[Pierre-Marie de Rodat]

This patch ensures that an abstract state declared with simple option
"synchronous" is automatically considered "external".

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

2018-05-21  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* einfo.adb (Is_External_State): An abstract state is also external
	when it is declared with option "synchronous".
	* einfo.ads: Update the documentation of synthesized attribute
	Is_External_State.
	* sem_util.adb (Find_Simple_Properties): New routine.
	(Is_Enabled_External_Property): New routine.
	(State_Has_Enabled_Property): Reimplemented. The two flavors of option
	External have precedence over option Synchronous when determining
	whether a property is in effect.

gcc/testsuite/

	* gnat.dg/sync2.adb, gnat.dg/sync2.ads: New testcase.
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