[Ada] Plug loophole for built-in-place return with limited_with clause

[Ada] Plug loophole for built-in-place return with limited_with clause

When the result type of a function requiring built-in-place return is
only visible through a limited_with clause, the compiled needs to wait
for the nonlimited view to be available in order to compute whether
the built-in-place return is needed, and this comprises tagging the
function with the Returns_By_Ref flag.

gcc/ada/

	* exp_ch6.adb (Build_In_Place_Formal): Also compute Returns_By_Ref
	for the function if the extra formals were not built initially.