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.

llvm-svn: 168996
diff --git a/llvm/unittests/Support/Path.cpp b/llvm/unittests/Support/Path.cpp
index 63c9ae0..38bad8f 100644
--- a/llvm/unittests/Support/Path.cpp
+++ b/llvm/unittests/Support/Path.cpp
@@ -375,7 +375,7 @@
   
   // Unmap temp file
 
-#if LLVM_USE_RVALUE_REFERENCES
+#if LLVM_HAS_RVALUE_REFERENCES
   fs::mapped_file_region m(Twine(TempPath),
                              fs::mapped_file_region::readonly,
                              0,