[Ada] Wrong column in sloc of "expect name" error for loop variant

[Ada] Wrong column in sloc of "expect name" error for loop variant

This patch corrects error messages printed when using the pragma Loop_Variant
without a named argument from having an incorrect column number in some cases.

2018-01-11  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Pragma:Pragma_Loop_Variant): Modify error
	message to be printed on the pragma argument identifier.

gcc/testsuite/

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

From-SVN: r256484