[ADT] Move CachedHashString to its own header in ADT, and rename to CachedHashStringRef.

Summary:
Reclaiming the name 'CachedHashString' will let us add a type with that
name that owns its value.

Reviewers: timshen

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25644

llvm-svn: 284434
diff --git a/llvm/unittests/MC/StringTableBuilderTest.cpp b/llvm/unittests/MC/StringTableBuilderTest.cpp
index 517ada5..f68350d 100644
--- a/llvm/unittests/MC/StringTableBuilderTest.cpp
+++ b/llvm/unittests/MC/StringTableBuilderTest.cpp
@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ADT/SmallString.h"
 #include "llvm/MC/StringTableBuilder.h"
 #include "llvm/Support/Endian.h"
 #include "gtest/gtest.h"