Fix uninitialized variable with nested variant record types

Fix uninitialized variable with nested variant record types

This fixes a wrong code issue with nested variant record types: the
compiler generates move instructions that depend on an uninitialized
variable, which was initially a SAVE_EXPR not instantiated early enough.

gcc/ada/ChangeLog:
	* gcc-interface/decl.c (build_subst_list): For a definition, make
	sure to instantiate the SAVE_EXPRs generated by the elaboration of
	the constraints in front of the elaboration of the type itself.

gcc/testsuite/ChangeLog:
	* gnat.dg/discr59.adb: New test.
	* gnat.dg/discr59_pkg1.ads: New helper.
	* gnat.dg/discr59_pkg2.ads: Likewise.