[Ada] Add note about gnatcov reliance on thunk names

[Pierre-Marie de Rodat]

Add a note indicating that the names generated for interface thunks
should not be changed without reflecting the change in gnatcoverage, as
it relies on that information to exclude thunks from source coverage.

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

2020-06-05  Thomas Quinot  <qui...@adacore.com>

gcc/ada/

        * exp_disp.adb (Expand_Interface_Thunk): Add comment about
        gnatcov reliance on specific name used for thunks.
--- gcc/ada/exp_disp.adb
+++ gcc/ada/exp_disp.adb
@@ -2137,6 +2137,11 @@ package body Exp_Disp is
       end loop;
 
       Thunk_Id := Make_Temporary (Loc, 'T');
+
+      --  Note: any change to this symbol name needs to be coordinated
+      --  with GNATcoverage, as that tool relies on it to identify
+      --  thunks and exclude them from source coverage analysis.
+
       Set_Ekind (Thunk_Id, Ekind (Prim));
       Set_Is_Thunk (Thunk_Id);
       Set_Convention (Thunk_Id, Convention (Prim));