[IntervalTree] Initialize find_iterator::Point

There was initially a msan report for use-of-uninitialized value due to
a bug in https://reviews.llvm.org/D138526. find_iterator::Point is
uninitialized for the default constructor of find_iterator, which is
used by IntervalTree::end. This change is not required, but its good
practice to make sure all class members are initialized.

Differential Revision: https://reviews.llvm.org/D149698