[NFC] Header cleanup

Removed some unused headers, replaced some headers with forward class declarations.

Found using simple scripts like this one:
clear && ack --cpp -l '#include "llvm/ADT/IndexedMap.h"' | xargs grep -L 'IndexedMap[<]' | xargs grep -n --color=auto 'IndexedMap'

Patch by Eugene Kosov <claprix@yandex.ru>

Differential Revision: http://reviews.llvm.org/D19219

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266595
diff --git a/llvm/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp b/llvm/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp
index 3f80e45..b21149c 100644
--- a/llvm/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp
+++ b/llvm/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp
@@ -9,13 +9,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "FuzzerInterface.h"
 #include "llvm-c/Disassembler.h"
 #include "llvm-c/Target.h"
-#include "llvm/ADT/ArrayRef.h"
 #include "llvm/MC/SubtargetFeature.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/raw_ostream.h"
-#include "FuzzerInterface.h"
 
 using namespace llvm;