[Ada] SPARK pointer support extended to local borrowers and observers

[Pierre-Marie de Rodat]

SPARK rules allow local borrowers and observers to be declared. During
their lifetime, the access to the borrowed/observed object is
restricted.

There is no impact on compilation.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-07-03  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_spark.adb: Add support for locally borrowing and observing
	a path.
	(Get_Root_Object): Add parameter Through_Traversal to denote
	when we are interesting in getting to the traversed parameter.
	(Is_Prefix_Or_Almost): New function to support detection of
	illegal access to borrowed or observed paths.
	(Check_Pragma): Add analysis of assertion pragmas.
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