[Ada] Fix comment about subprogram unnesting and unconstrained arrays

[Pierre-Marie de Rodat via Gcc-patches]

The original code for subprogram unnesting used 'Access for
unconstrained arrays. This was recently changed to 'Unchecked_Access for
GNAT-to-LLVM, but a reference to 'Access still appears in the comment.

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

gcc/ada/

        * exp_unst.adb (Unnest_Subprogram): Sync comment with the
        current code.
diff --git a/gcc/ada/exp_unst.adb b/gcc/ada/exp_unst.adb
--- a/gcc/ada/exp_unst.adb
+++ b/gcc/ada/exp_unst.adb
@@ -2093,7 +2093,8 @@ package body Exp_Unst is
 
                                  --  Build and insert the assignment:
                                  --    ARECn.nam := nam'Address
-                                 --  or else 'Access for unconstrained array
+                                 --  or else 'Unchecked_Access for
+                                 --  unconstrained array.
 
                                  if Needs_Fat_Pointer (Ent) then
                                     Attr := Name_Unchecked_Access;