[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-diff/DiffConsumer.h b/llvm/tools/llvm-diff/DiffConsumer.h
index 36d4a12..82f5ce5 100644
--- a/llvm/tools/llvm-diff/DiffConsumer.h
+++ b/llvm/tools/llvm-diff/DiffConsumer.h
@@ -17,12 +17,12 @@
 #include "DiffLog.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/StringRef.h"
 #include "llvm/IR/Value.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace llvm {
+class StringRef;
   class Module;
   class Value;
   class Function;
diff --git a/llvm/tools/llvm-diff/DiffLog.cpp b/llvm/tools/llvm-diff/DiffLog.cpp
index ed86058f..898749e 100644
--- a/llvm/tools/llvm-diff/DiffLog.cpp
+++ b/llvm/tools/llvm-diff/DiffLog.cpp
@@ -13,7 +13,6 @@
 
 #include "DiffLog.h"
 #include "DiffConsumer.h"
-#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/IR/Instructions.h"
 
diff --git a/llvm/tools/llvm-diff/DifferenceEngine.cpp b/llvm/tools/llvm-diff/DifferenceEngine.cpp
index fc9ea99..df208a2 100644
--- a/llvm/tools/llvm-diff/DifferenceEngine.cpp
+++ b/llvm/tools/llvm-diff/DifferenceEngine.cpp
@@ -16,7 +16,6 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/CallSite.h"
diff --git a/llvm/tools/llvm-diff/DifferenceEngine.h b/llvm/tools/llvm-diff/DifferenceEngine.h
index f0d8311..7f084a3 100644
--- a/llvm/tools/llvm-diff/DifferenceEngine.h
+++ b/llvm/tools/llvm-diff/DifferenceEngine.h
@@ -17,7 +17,6 @@
 
 #include "DiffConsumer.h"
 #include "DiffLog.h"
-#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include <utility>
 
diff --git a/llvm/tools/llvm-diff/llvm-diff.cpp b/llvm/tools/llvm-diff/llvm-diff.cpp
index ae58f5c..e449d69 100644
--- a/llvm/tools/llvm-diff/llvm-diff.cpp
+++ b/llvm/tools/llvm-diff/llvm-diff.cpp
@@ -13,8 +13,6 @@
 
 #include "DiffLog.h"
 #include "DifferenceEngine.h"
-#include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"