Reorganise the release-dependent parts of common/.

The previous solution (dba65e4) splattered Image-related units into
every release subdirectory; all were the same until GCC12.

The revised solution moves the GCC12-related units into
common/post-gcc12, the others into pre-gcc12.

Issue #40 (GNAT deciding that parens for aggregates are obsolescent in
spite of not being in Annex J, and issuing warnings with -gnatwa) is
addressed by moving the Container units into these subdirectories and
editing the ones in post-gcc12/ to use [brackets] rather than
(parentheses).

  * common/common.gpr (Image_Path): new, chooses post-gcc12 or pre-gcc12.
    (Paths): include Image_Path.
  * common/post-gcc12/a-chtgbo.adb: use brackets in aggregates.
  * common/post-gcc12/a-cobove.adb: likewise.
  * common/post-gcc12/a-cohata.ads: likewise.

common/common.gpr

@@ -21,6 +21,9 @@ abstract project Common is
    type Compiler_Release is
      ("gcc6", "gnat-gpl-2017", "gcc7", "gcc8", "gnat-ce-2020",
       "gcc11", "gcc12");
+
+   --  We need to cope with changes to tasking and tag code between
+   --  compiler releases.
    Release : Compiler_Release := external ("RELEASE", "gcc11");
    Release_Path := Release;
    case Release is
@@ -29,12 +32,22 @@ abstract project Common is
       when others =>
          null;
    end case;
+   --  We also need to cope with the Ada2022 changes to 'Image code
+   --  and the new aggregate syntax, which is the same up to GCC12.
+   Image_Path := "";
+   case Release is
+      when "gcc12" =>
+         Image_Path := "post-gcc12";
+      when others =>
+         Image_Path := "pre-gcc12";
+   end case;
 
    type Install_Locally is ("yes", "no");
    Local : Install_Locally := external ("INSTALL_LOCALLY", "yes");
 
    Paths := (project'Project_Dir,
              project'Project_Dir & "math",
-             project'Project_Dir & Release_Path);
+             project'Project_Dir & Release_Path,
+             project'Project_Dir & Image_Path);
 
 end Common;