ada: Fix printing boolean attributes in the SARIF report

Boolean attributes should have the value true or false
without any quotes.

gcc/ada/ChangeLog:

	* diagnostics-json_utils.adb: Add new method
	Write_Boolean_Attribute.
	* diagnostics-json_utils.ads: Likewise.
	* diagnostics-sarif_emitter.adb (Print_Invocations): print
	the executionSuccesful property value without extra quotes.