[Ada] Use static discriminant value for discriminated task record

[Ada] Use static discriminant value for discriminated task record

This patch allows the construction of a static subtype for the generated
constrained Secondary_Stack component of a task for which a stack size
is specified, when compiling for a restricted run-time that forbids
dynamic allocation. Needed for LLVM.

2019-09-18  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch3.adb (Constrain_Component_Type): For a discriminated
	type, handle the case of a constraint given by a conversion of a
	discriminant of the enclosing type. Necessary when compiling a
	discriminated task for a restricted run-time, when the generated
	Secondary_Stack component may be set by means of an aspect on
	the task type.

From-SVN: r275863