[C++11] Replace some comparisons with 'nullptr' with simple boolean checks to reduce verbosity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205829 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/StringRef.h

@@ -186,7 +186,7 @@ namespace llvm {
 
     /// str - Get the contents as an std::string.
     std::string str() const {
-      if (Data == nullptr) return std::string();
+      if (!Data) return std::string();
       return std::string(Data, Length);
     }