Add operator- to Path's reverse_iterator. Needed for D19666

include/llvm/Support/Path.h

class reverse_iterator
   friend reverse_iterator rend(StringRef path);
 
 public:
   reference operator*() const { return Component; }
   pointer   operator->() const { return &Component; }
   reverse_iterator &operator++();    // preincrement
   bool operator==(const reverse_iterator &RHS) const;
   bool operator!=(const reverse_iterator &RHS) const { return !(*this == RHS); }
+
+  /// @brief Difference in bytes between this and RHS.
+  ptrdiff_t operator-(const reverse_iterator &RHS) const;
 };
 
 /// @brief Get begin iterator over \a path.
 /// @param path Input path.
 /// @returns Iterator initialized with the first component of \a path.
 const_iterator begin(StringRef path);
 
 /// @brief Get end iterator over \a path.

lib/Support/Path.cpp

reverse_iterator &reverse_iterator::operator++() {
   return *this;
 }
 
 bool reverse_iterator::operator==(const reverse_iterator &RHS) const {
   return Path.begin() == RHS.Path.begin() && Component == RHS.Component &&
          Position == RHS.Position;
 }
 
+ptrdiff_t reverse_iterator::operator-(const reverse_iterator &RHS) const {
+  return Position - RHS.Position;
+}
+
 StringRef root_path(StringRef path) {
   const_iterator b = begin(path),
                  pos = b,
                  e = end(path);
   if (b != e) {
     bool has_net = b->size() > 2 && is_separator((*b)[0]) && (*b)[1] == (*b)[0];
     bool has_drive =
 #ifdef LLVM_ON_WIN32