[support] Add StringRef::withNullAsEmpty

[zturner]

Summary

This almost doesn't seem worthy of review, but since StringRef is kinda fundamental, I'll throw it up just in case.

llvm/trunk/include/llvm/ADT/StringRef.h

/*implicit*/ StringRef(const char *data, size_t length)
         "StringRef cannot be built from a NULL argument with non-null length");
       }
 
     /// Construct a string ref from an std::string.
     LLVM_ATTRIBUTE_ALWAYS_INLINE
     /*implicit*/ StringRef(const std::string &Str)
       : Data(Str.data()), Length(Str.length()) {}
 
+    static StringRef withNullAsEmpty(const char *data) {
+      return StringRef(data ? data : "");
+    }
+
     /// @}
     /// @name Iterators
     /// @{
 
     iterator begin() const { return Data; }
 
     iterator end() const { return Data + Length; }