[Ada] Unnesting: fix handling of private types that are synchronized

[Ada] Unnesting: fix handling of private types that are synchronized

2018-10-09  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_unst.adb (In_Synchronized_Call): Handle properly private
	types whose full views are synchronized types, in order to
	recognize generated subprograms whose bodies must be considered
	reachable even if no direct calls to them are visible in the
	source.

From-SVN: r264974