[llvm] [MC] Forward declare ELFObjectWriter (PR #100989)

[Matt Arsenault via llvm-commits]

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@@ -1,13 +1,20 @@
-#include "AVRELFStreamer.h"
+//===--- AVRELFStreamer.cpp - AVR Target Streamer -------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
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arsenm wrote:

Unrelated change but ok 

https://github.com/llvm/llvm-project/pull/100989