ada: Run-time error with GNAT-LLVM on container aggregate with finalization

When unnesting is enabled, the compiler was failing to copy the At_End_Proc
field from a block statement to the procedure created to replace it when
unnesting of top-level blocks is done.  At run time this could lead to
exceptions due to missing finalization calls.

gcc/ada/

	* exp_ch7.adb (Unnest_Block): Copy the At_End_Proc from the block
	statement to the newly created subprogram body.