Add front() method to SetVector.

[jsjodin]

Summary

This will later be needed by the Machine CFG Structurizer.

[• tstellarAMD]

LGTM.

llvm/trunk/include/llvm/ADT/SetVector.h

reverse_iterator rend() {
     return vector_.rend();
   }
 
   /// \brief Get a const_reverse_iterator to the beginning of the SetVector.
   const_reverse_iterator rend() const {
     return vector_.rend();
   }
 
+  /// \brief Return the first element of the SetVector.
+  const T &front() const {
+    assert(!empty() && "Cannot call front() on empty SetVector!");
+    return vector_.front();
+  }
+
   /// \brief Return the last element of the SetVector.
   const T &back() const {
     assert(!empty() && "Cannot call back() on empty SetVector!");
     return vector_.back();
   }
 
   /// \brief Index into the SetVector.
   const_reference operator[](size_type n) const {