[Ada] Ineffective use warning is suppressed when performing verification

[Ada] Ineffective use warning is suppressed when performing verification

This patch fixes an issue whereby the compiler incorrectly marked use clauses
as effective due to code generated for verification referencing certain types
leading to missing use clause warnings.

No reasonably small testcase available.

2018-05-24  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Entity_Name): Add guard to protect against
	marking use clauses as effective when the reference appears within
	generated code.

From-SVN: r260652