[Ada] Compiler loop on expression function and predicate in generic unit

[Ada] Compiler loop on expression function and predicate in generic unit

This patch fixes an infinite loop in the compiler when analyzing an
expression function whose expression mentions a subtype with a static
predicate, and the context is a generic unit.

2018-05-25  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch13.adb (Build_Predicate_Functions): The predicate function
	declaration is inserted into the tree and analyzed at that point, so
	should not be reinserted when the body is constructed. Inside a
	generic, ensure that the body is not inserted twice in the tree.

gcc/testsuite/

	* gnat.dg/static_pred1.adb, gnat.dg/static_pred1.ads: New testcase.

From-SVN: r260716