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/APSInt.cpp b/llvm/lib/Support/APSInt.cpp
index 975457c..46c0f70 100644
--- a/llvm/lib/Support/APSInt.cpp
+++ b/llvm/lib/Support/APSInt.cpp
@@ -14,6 +14,7 @@
 
 #include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/FoldingSet.h"
+#include "llvm/ADT/StringRef.h"
 
 using namespace llvm;