[Ada] Allow reading a borrowed object inside a call to a pledge function

[Ada] Allow reading a borrowed object inside a call to a pledge function

No impact on regular compilation.

2019-08-19  Claire Dross  <dross@adacore.com>

gcc/ada/

	* sem_spark.ads, sem_spark.adb (Is_Pledge_Function): New
	parameter of the generic. Function used to decide whether a
	function is a pledge function.
	(Check_Not_Borrowed): Disable check inside the second parameter
	of a pledge function for the path borrowed by the first
	parameter. Also disable checks for entities inside a Global
	contract.

From-SVN: r274644