[StringRef] Use default member initializers and = default.

[malcolm.parsons]

Summary

This makes the default constructor implicitly constexpr and noexcept.

[malcolm.parsons]

Upload the correct patch.

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

namespace llvm {
   public:
     typedef const char *iterator;
     typedef const char *const_iterator;
     static const size_t npos = ~size_t(0);
     typedef size_t size_type;
 
   private:
     /// The start of the string, in an external buffer.
-    const char *Data;
+    const char *Data = nullptr;
 
     /// The length of the string.
-    size_t Length;
+    size_t Length = 0;
 
     // Workaround memcmp issue with null pointers (undefined behavior)
     // by providing a specialized version
     LLVM_ATTRIBUTE_ALWAYS_INLINE
     static int compareMemory(const char *Lhs, const char *Rhs, size_t Length) {
       if (Length == 0) { return 0; }
       return ::memcmp(Lhs,Rhs,Length);
     }
 
   public:
     /// @name Constructors
     /// @{
 
     /// Construct an empty string ref.
-    /*implicit*/ StringRef() : Data(nullptr), Length(0) {}
+    /*implicit*/ StringRef() = default;
 
     /// Disable conversion from nullptr.  This prevents things like
     /// if (S == nullptr)
     StringRef(std::nullptr_t) = delete;
 
     /// Construct a string ref from a cstring.
     LLVM_ATTRIBUTE_ALWAYS_INLINE
     /*implicit*/ StringRef(const char *Str)