Add some utilities to iterator_range for trimming a range and constructing one from a container.

To be used in clang in a follow-up commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242823 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/iterator_range.h

@@ -20,6 +20,7 @@
 #define LLVM_ADT_ITERATOR_RANGE_H
 
 #include <utility>
+#include <iterator>
 
 namespace llvm {
 
@@ -32,6 +33,12 @@ class iterator_range {
   IteratorT begin_iterator, end_iterator;
 
 public:
+  //TODO: Add SFINAE to test that the Container's iterators match the range's
+  //      iterators.
+  template <typename Container>
+  iterator_range(Container &&c)
+  //TODO: Consider ADL/non-member begin/end calls.
+      : begin_iterator(c.begin()), end_iterator(c.end()) {}
   iterator_range(IteratorT begin_iterator, IteratorT end_iterator)
       : begin_iterator(std::move(begin_iterator)),
         end_iterator(std::move(end_iterator)) {}
@@ -51,6 +58,11 @@ template <class T> iterator_range<T> make_range(T x, T y) {
 template <typename T> iterator_range<T> make_range(std::pair<T, T> p) {
   return iterator_range<T>(std::move(p.first), std::move(p.second));
 }
+
+template<typename T>
+iterator_range<decltype(begin(std::declval<T>()))> drop_begin(T &&t, int n) {
+  return make_range(std::next(begin(t), n), end(t));
+}
 }
 
 #endif