Fix build breakage due to typo in cast.

llvm-svn: 282183
diff --git a/llvm/unittests/ADT/StringRefTest.cpp b/llvm/unittests/ADT/StringRefTest.cpp
index 40ab4e0..39ed71b 100644
--- a/llvm/unittests/ADT/StringRefTest.cpp
+++ b/llvm/unittests/ADT/StringRefTest.cpp
@@ -580,6 +580,8 @@
   , "08"                    // illegal oct characters
   , "0o8"                   // illegal oct characters
   , "-123"                  // negative unsigned value
+  , "0x"
+  , "0b"
 };