Don't rely on the default constructor default constructing a begin and

end iterator for iterator_range<>. I removed this constructor because
for some iterators (notably pointers) it left begin and end
uninitialized. It also is an usual constraint that an iterator default
constructs to a valid end iterator such that the pair of them for
a valid range. In the three places where this was used in Clang,
explicitly build the empty range from the iterators and comment on why
default constructed iterators make sense here.

llvm-svn: 225594

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