Remove some unneeded headers and replace some headers with forward class declarations (NFC)

Differential Revision: http://reviews.llvm.org/D19154

Patch by Eugene Kosov <claprix@yandex.ru>

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266524
diff --git a/llvm/lib/Support/ScaledNumber.cpp b/llvm/lib/Support/ScaledNumber.cpp
index 987c2d8..b9432d4 100644
--- a/llvm/lib/Support/ScaledNumber.cpp
+++ b/llvm/lib/Support/ScaledNumber.cpp
@@ -13,6 +13,7 @@
 
 #include "llvm/Support/ScaledNumber.h"
 #include "llvm/ADT/APFloat.h"
+#include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"