[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

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -28,12 +28,11 @@
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/UseListOrder.h"
 #include "llvm/IR/ValueSymbolTable.h"
-#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/Program.h"
-#include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/SHA1.h"
+#include "llvm/Support/raw_ostream.h"
 #include <cctype>
 #include <map>
 using namespace llvm;