[Ada] Spurious ineffective use_clause warning

[Ada] Spurious ineffective use_clause warning

This patch fixes an issue whereby user-defined subprograms used as
generic actuals with corresponding formals containing other formal types
led to spurious ineffective use_clause warnings.

2018-09-26  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_ch8.adb (Analyze_Subprogram_Renaming): Add extra condition
	to check for unmarked subprogram references coming from
	renamings.

gcc/testsuite/

	* gnat.dg/warn16.adb: New testcase.

From-SVN: r264635