[Ada] Support chained calls to traversal functions in SPARK

[Ada] Support chained calls to traversal functions in SPARK

This change only affects the SPARK toolset. In the part of semantic
analysis enforcing ownership rules for SPARK, it corrects a crash in
analysis of a declaration of a local borrower whose definition is a
chain of several calls to traversal functions.

2019-09-17  Claire Dross  <dross@adacore.com>

gcc/ada/

	* sem_spark.adb (Get_Observed_Or_Borrowed_Expr): If the
	definition of a local borrower contains calls to traversal
	functions, the borrowed expression is the first parameter of the
	first traversal function call in the definition.

From-SVN: r275785