Import llvm::SmallString and dependencies.

Update StringRef and StringExtras with SmallVector functions.

include/llvm/StringRef.h

@@ -18,6 +18,8 @@
 #include <utility>
 
 namespace llvm {
+  template <typename T>
+  class SmallVectorImpl;
   class StringRef;
 
   /// Helper functions for StringRef::getAsInteger.
@@ -434,6 +436,24 @@ namespace llvm {
       return std::make_pair(slice(0, Idx), slice(Idx + Separator.size(), npos));
     }
 
+    /// Split into substrings around the occurrences of a separator string.
+    ///
+    /// Each substring is stored in \p A. If \p MaxSplit is >= 0, at most
+    /// \p MaxSplit splits are done and consequently <= \p MaxSplit
+    /// elements are added to A.
+    /// If \p KeepEmpty is false, empty strings are not added to \p A. They
+    /// still count when considering \p MaxSplit
+    /// An useful invariant is that
+    /// Separator.join(A) == *this if MaxSplit == -1 and KeepEmpty == true
+    ///
+    /// \param A - Where to put the substrings.
+    /// \param Separator - The string to split on.
+    /// \param MaxSplit - The maximum number of times the string is split.
+    /// \param KeepEmpty - True if empty substring should be added.
+    void split(SmallVectorImpl<StringRef> &A,
+               StringRef Separator, int MaxSplit = -1,
+               bool KeepEmpty = true) const;
+
     /// Split into two substrings around the last occurrence of a separator
     /// character.
     ///