[Ada] Spurious ineffective use_clause warning

[Ada] Spurious ineffective use_clause warning

This patch fixes an issue whereby expansion of post conditions may lead
to spurious ineffective use_clause warnings when a use type clause is
present in a package specification and a use package clause exists in
the package body on the package containing said type.

2019-09-18  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_ch8.adb (Use_One_Type): Add guard to prevent warning on a
	reundant use package clause where there is no previous
	use_clause in the chain.

gcc/testsuite/

	* gnat.dg/warn30.adb, gnat.dg/warn30.ads: New testcase.

From-SVN: r275861