[Ada] Fix check on placement of multiple loop (in)variant pragmas

[Ada] Fix check on placement of multiple loop (in)variant pragmas

Loop (in)variants should appear next to each other, which is checked by GNAT
frontend. As statements inserted during expansion may break this contiguity,
GNAT recognizes specially such statements which originate in loop pragmas. In
some cases, this special treatment was not properly put in place, which lead to
spurious errors being issued.

2018-05-31  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Pragma.Check_Loop_Pragma_Placement): Inverse
	order of treatment between nodes recognized as loop pragmas (or
	generated from one) and block statements.

From-SVN: r260996