[SaveAndRestore] Upgrade this to support non-copyable types.

[lattner]

Summary

This adds a constructor and upgrades the dtor to work with
move-only types.

[lattner]

This is an obvious patch and tested with out of tree clients. There isn't an obvious owner for this file at this time besides me :-/, anyone object to landing this?

[bzcheeseman]

LGTM - I might clarify get to be getOldValue or something to make it easier on the reader at the callsite, but I don't have strong feelings about it.

[lattner]

agreed, I'm not in love with the existing API, but I don't want to break everyone for no reason. Thanks for the review!

[rriddle]

This LGTM.

llvm/include/llvm/Support/SaveAndRestore.h

 #ifndef LLVM_SUPPORT_SAVEANDRESTORE_H
 #define LLVM_SUPPORT_SAVEANDRESTORE_H
 
 namespace llvm {
 
 /// A utility class that uses RAII to save and restore the value of a variable.
 template <typename T> struct SaveAndRestore {
   SaveAndRestore(T &X) : X(X), OldValue(X) {}
-  SaveAndRestore(T &X, const T &NewValue) : X(X), OldValue(X) {
-    X = NewValue;
+  SaveAndRestore(T &X, const T &NewValue) : X(X), OldValue(X) { X = NewValue; }
+  SaveAndRestore(T &X, T &&NewValue) : X(X), OldValue(std::move(X)) {
+    X = std::move(NewValue);
   }
-  ~SaveAndRestore() { X = OldValue; }
-  T get() { return OldValue; }
+  ~SaveAndRestore() { X = std::move(OldValue); }
+  const T &get() { return OldValue; }
 
 private:
   T &X;
   T OldValue;
 };
 
 } // namespace llvm
 
 #endif