[ADT] Remove the unused default constructor for iterator_range.

This default constructor is a bit weird. It left the range in an invalid
state. That might be reasonable so that you can construct a local
iterator range and assign to it based on some logic to compute the range
you want. If folks would like to support that use case, I can add it
back, but in 238-odd usages none have actually wanted to do this. ;]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225592 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/iterator_range.h

@@ -32,7 +32,6 @@ class iterator_range {
   IteratorT begin_iterator, end_iterator;
 
 public:
-  iterator_range() {}
   iterator_range(IteratorT begin_iterator, IteratorT end_iterator)
       : begin_iterator(std::move(begin_iterator)),
         end_iterator(std::move(end_iterator)) {}