[CodeCompletion] Make all result's string fields null terminated

This is convenient for clients to pass these fields values to C
functions. Introduce NullTerminatedStringRef to guarantee that.

include/swift/Basic/StringExtras.h

@@ -469,6 +469,52 @@ bool omitNeedlessWords(StringRef &baseName,
 /// If the name has a completion-handler suffix, strip off that suffix.
 Optional<StringRef> stripWithCompletionHandlerSuffix(StringRef name);
 
+/// Represents a string that can be efficiently retrieved either as a StringRef
+/// or as a null-terminated C string.
+class NullTerminatedStringRef {
+  StringRef Ref;
+
+public:
+  /// Create a \c NullTerminatedStringRef from a null-terminated C string with
+  /// size \p Size (excluding the null character).
+  NullTerminatedStringRef(const char *Data, size_t Size) : Ref(Data, Size) {
+    assert(Data != nullptr && Data[Size] == '\0' &&
+           "Data should be null-terminated");
+  }
+
+  /// Create an empty null-terminated string. \c data() is not a \c nullptr.
+  constexpr NullTerminatedStringRef() : Ref("") {}
+
+  /// Create an null terminated string with a C string.
+  constexpr NullTerminatedStringRef(const char *Data) : Ref(Data) {}
+
+  /// Create a null-terminated string, copying \p Str into \p A .
+  template <typename Allocator>
+  NullTerminatedStringRef(StringRef Str, Allocator &A) : Ref("") {
+    if (Str.empty())
+      return;
+
+    size_t size = Str.size();
+    char *memory = A.template Allocate<char>(size + 1);
+    memcpy(memory, Str.data(), size);
+    memory[size] = '\0';
+    Ref = {memory, size};
+  }
+
+  /// Returns the string as a `StringRef`. The `StringRef` does not include the
+  /// null character.
+  operator StringRef() const { return Ref; }
+
+  /// Returns the string as a null-terminated C string.
+  const char *data() const { return Ref.data(); }
+
+  /// The size of the string, excluding the null character.
+  size_t size() const { return Ref.size(); }
+
+  bool empty() const { return Ref.empty(); }
+  int compare(NullTerminatedStringRef RHS) const { return Ref.compare(RHS); }
+};
+
 } // end namespace swift
 
 #endif // SWIFT_BASIC_STRINGEXTRAS_H