[Ada] Use declared type for deciding on SPARK pointer rules

[Ada] Use declared type for deciding on SPARK pointer rules

A constant of pointer type is considered as mutable in SPARK, according
to SPARK RM 3.10, but this should be based on the declared type of the
constant instead of its underlying type.

There is no impact on compilation hence no test.

2019-09-19  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Depends_In_Decl_Part): Simplify previous
	test.

From-SVN: r275950