C++ PATCH for c++/49568 (unneeded thunks emitted on xcoff target)

[Jason Merrill <jason@redhat.com>]

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This issue was exposed by honza's change to how thunks are represented, 
but the underlying problem was that we weren't setting DECL_COMDAT on 
them properly.

Tested that the bug is fixed with an alpha-dec-osf5.1b cross compiler, 
regression tested x86_64-pc-linux-gnu, applying to trunk.

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commit d8a77afc071a28ee411d484793b90aaf6959cc5f
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Jul 6 16:28:34 2011 -0400

    	PR c++/49568
    	* method.c (make_thunk, use_thunk): Copy DECL_COMDAT.

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index 9b9eb9a..d41a4dd 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -140,11 +140,12 @@ make_thunk (tree function, bool this_adjusting,
   THUNK_VIRTUAL_OFFSET (thunk) = virtual_offset;
   THUNK_ALIAS (thunk) = NULL_TREE;
 
-  /* The thunk itself is not a constructor or destructor, even if
-     the thing it is thunking to is.  */
   DECL_INTERFACE_KNOWN (thunk) = 1;
   DECL_NOT_REALLY_EXTERN (thunk) = 1;
+  DECL_COMDAT (thunk) = DECL_COMDAT (function);
   DECL_SAVED_FUNCTION_DATA (thunk) = NULL;
+  /* The thunk itself is not a constructor or destructor, even if
+     the thing it is thunking to is.  */
   DECL_DESTRUCTOR_P (thunk) = 0;
   DECL_CONSTRUCTOR_P (thunk) = 0;
   DECL_EXTERNAL (thunk) = 1;
@@ -342,6 +343,7 @@ use_thunk (tree thunk_fndecl, bool emit_p)
   DECL_VISIBILITY (thunk_fndecl) = DECL_VISIBILITY (function);
   DECL_VISIBILITY_SPECIFIED (thunk_fndecl)
     = DECL_VISIBILITY_SPECIFIED (function);
+  DECL_COMDAT (thunk_fndecl) = DECL_COMDAT (function);
   if (DECL_ONE_ONLY (function) || DECL_WEAK (function))
     make_decl_one_only (thunk_fndecl, cxx_comdat_group (thunk_fndecl));