Avoid SmallString.h include in MD5.h, NFC

Saves 200 includes, which is mostly immaterial.

llvm/include/llvm/Support/MD5.h

@@ -28,7 +28,6 @@
 #ifndef LLVM_SUPPORT_MD5_H
 #define LLVM_SUPPORT_MD5_H
 
-#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Endian.h"
 #include <array>
@@ -36,6 +35,7 @@
 
 namespace llvm {
 
+template <unsigned N> class SmallString;
 template <typename T> class ArrayRef;
 
 class MD5 {

llvm/lib/Support/MD5.cpp

@@ -39,6 +39,7 @@
 
 #include "llvm/Support/MD5.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Format.h"

llvm/lib/Target/AMDGPU/AMDGPUOpenCLEnqueuedBlockLowering.cpp

@@ -33,6 +33,7 @@
 
 #include "AMDGPU.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -18,6 +18,7 @@
 
 #include "CoroInternal.h"
 #include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/Analysis/PtrUseVisitor.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Config/llvm-config.h"

llvm/tools/llvm-diff/DifferenceEngine.cpp

@@ -14,6 +14,7 @@
 #include "DifferenceEngine.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/IR/CFG.h"