[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/unittests/Support/ConvertUTFTest.cpp b/llvm/unittests/Support/ConvertUTFTest.cpp
index 61ed252..0af09e9 100644
--- a/llvm/unittests/Support/ConvertUTFTest.cpp
+++ b/llvm/unittests/Support/ConvertUTFTest.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/ConvertUTF.h"
+#include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/Format.h"
 #include "gtest/gtest.h"
 #include <string>