[Ada] Detect misplaced assertions between loop invariants

[Ada] Detect misplaced assertions between loop invariants

Loop invariants and loop variants should all be colocated, as defined in
SPARK RM 5.5.3(8). The code checking that rule was incorrectly accepting
pragma Assert between two loop invariants. Now fixed.

2018-05-25  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_prag.adb (Check_Grouping): Modify test to ignore statements and
	declarations not coming from source.

From-SVN: r260715