Ada: Fix too large secondary stack allocation for aggregate return

This is a regression present on the mainline and 15 branch.  The problematic
allocation may occur when the result type of the function is a discriminated
record type with defaulted discriminants.  Although it had been present for
a long time when the type is limited, the problem was recently propagated
to nonlimited types because of an optimization.

While the fix is a one-liner, the change also makes it so that SS_Allocate
raises a Storage_Error when the size overflows, like the other allocators.

gcc/ada/
	* exp_ch6.adb (Expand_Simple_Function_Return): Use a constant return
	object when the simple return is rewritten as an extended one.
	* libgnat/s-secsta.adb (SS_Allocate): Raise Storage_Error if the
	requested size is negative.
	* libgnat/s-secsta__cheri.adb (SS_Allocate): Likewise.

gcc/testsuite/
	* gnat.dg/aggr35.adb: New test.
	* gnat.dg/aggr35_pkg.ads, gnat.dg/aggr35_pkg.adb: New helper.