[Ada] Minor refactoring

2019-07-11  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* checks.adb: Fix typo in comment.
	* sem.adb (Semantics): Avoid repeated calls to
	In_Extended_Main_Source_Unit by reusing an already-cached
	result.
	* sem_util.adb (First_Global): Fix style.

From-SVN: r273406