String concatenation can be implemented either in-line or out-of-line by the
compiler, depending on the optimization level and other factors.  But doing
in-line concatenation at library level is undesirable in general and the
compiler already avoids it for simple declarations:

  S : String := S1 & S2;  -- out-of-line at all optimization levels

but not for slightly more complex ones:

  S : String := Ada.Characters.Handling.To_Upper (S1 & S2);

which are implemented in-line for -O1 and above.

This patch changes the second case to using out-of-line concatenation at all
optimization levels which, among other things, generates more compact code.

The following package must always use out-of-line concatenation:

with Ada.Characters.Handling;

package P is

  Scope: constant String := Ada.Characters.Handling.To_Upper ("P");

  function Full_String return String;

end P;

package body P is

  Full_Scope : constant String
    := Ada.Characters.Handling.To_Upper (Scope & ".body");

  function Full_String return String is (Full_Scope);

end P;

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

2017-04-25  Eric Botcazou  <ebotcazou@adacore.com>

	* exp_ch4.adb (Library_Level_Target): New function.
	(Expand_Concatenate): When optimization is enabled, also expand
	the operation out-of-line if the concatenation is present within
	the expression of the declaration of a library-level object and
	not only if it is the expression of the declaration.