[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/lib/CodeGen/LiveDebugVariables.h b/llvm/lib/CodeGen/LiveDebugVariables.h
index 3d36f4d2..afe87a5 100644
--- a/llvm/lib/CodeGen/LiveDebugVariables.h
+++ b/llvm/lib/CodeGen/LiveDebugVariables.h
@@ -21,12 +21,12 @@
 #ifndef LLVM_LIB_CODEGEN_LIVEDEBUGVARIABLES_H
 #define LLVM_LIB_CODEGEN_LIVEDEBUGVARIABLES_H
 
-#include "llvm/ADT/ArrayRef.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/IR/DebugInfo.h"
 
 namespace llvm {
 
+template <typename T> class ArrayRef;
 class LiveInterval;
 class LiveIntervals;
 class VirtRegMap;