Always using the primary stack when concatenating arrays can lead to a
very large stack usage. To alleviate this, if the current scope is
already using the secondary stack, then allocate the temporary object on
the secondary stack as well.

For the time being, this is for string concatenation, since using it
with e.g. controlled objects can lead to missing finalization and
memory leaks.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch4.adb (Expand_Concatenate): Allocate result of string
	concatenation on secondary stack when relevant.