Switch LLVM_USE_RVALUE_REFERENCES to LLVM_HAS_RVALUE_REFERENCES.

Rationale:
1) This was the name in the comment block. ;]
2) It matches Clang's __has_feature naming convention.
3) It matches other compiler-feature-test conventions.

Sorry for the noise. =]

I've also switch the comment block to use a \brief tag and not duplicate
the name.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168996 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/SmallBitVector.h

@@ -153,7 +153,7 @@ public:
       switchToLarge(new BitVector(*RHS.getPointer()));
   }
 
-#if LLVM_USE_RVALUE_REFERENCES
+#if LLVM_HAS_RVALUE_REFERENCES
   SmallBitVector(SmallBitVector &&RHS) : X(RHS.X) {
     RHS.X = 1;
   }
@@ -472,7 +472,7 @@ public:
     return *this;
   }
 
-#if LLVM_USE_RVALUE_REFERENCES
+#if LLVM_HAS_RVALUE_REFERENCES
   const SmallBitVector &operator=(SmallBitVector &&RHS) {
     if (this != &RHS) {
       clear();