[flang][NFC] Remove duplicated header file

[clementval]

Summary

Remove IntervalSet.h since it is a duplicate from
flang/include/flang/Lower/IntervalSet.h.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Lower/IntervalSet.h

-//===-- IntervalSet.h -------------------------------------------*- C++ -*-===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef FORTRAN_LOWER_INTERVALSET_H
-#define FORTRAN_LOWER_INTERVALSET_H
-
-#include <cassert>
-#include <map>
-
-namespace Fortran::lower {
-
-//===----------------------------------------------------------------------===//
-// Interval set
-//===----------------------------------------------------------------------===//
-
-/// Interval set to keep track of intervals, merging them when they overlap one
-/// another. Used to refine the pseudo-offset ranges of the front-end symbols
-/// into groups of aliasing variables.
-struct IntervalSet {
-  using MAP = std::map<std::size_t, std::size_t>;
-  using Iterator = MAP::const_iterator;
-
-  // Handles the merging of overlapping intervals correctly, efficiently.
-  void merge(std::size_t lo, std::size_t up) {
-    assert(lo <= up);
-    if (empty()) {
-      m.insert({lo, up});
-      return;
-    }
-    auto i = m.lower_bound(lo);
-    // i->first >= lo
-    if (i == begin()) {
-      if (up < i->first) {
-        // [lo..up] < i->first
-        m.insert({lo, up});
-        return;
-      }
-      // up >= i->first
-      if (i->second > up)
-        up = i->second;
-      fuse(lo, up, i);
-      return;
-    }
-    auto i1 = i;
-    if (i == end() || i->first > lo)
-      i = std::prev(i);
-    // i->first <= lo
-    if (i->second >= up) {
-      // i->first <= lo && up <= i->second, keep i
-      return;
-    }
-    // i->second < up
-    if (i->second < lo) {
-      if (i1 == end() || i1->first > up) {
-        // i < [lo..up] < i1
-        m.insert({lo, up});
-        return;
-      }
-      // i < [lo..up], i1->first <= up  -->  [lo..up] union [i1..?]
-      i = i1;
-    } else {
-      // i->first <= lo, lo <= i->second  -->  [i->first..up] union [i..?]
-      lo = i->first;
-    }
-    fuse(lo, up, i);
-  }
-
-  Iterator find(std::size_t pt) const {
-    auto i = m.lower_bound(pt);
-    if (i != end() && i->first == pt)
-      return i;
-    if (i == begin())
-      return end();
-    i = std::prev(i);
-    if (i->second < pt)
-      return end();
-    return i;
-  }
-
-  Iterator begin() const { return m.begin(); }
-  Iterator end() const { return m.end(); }
-  bool empty() const { return m.empty(); }
-  std::size_t size() const { return m.size(); }
-
-private:
-  // Find and fuse overlapping sets.
-  void fuse(std::size_t lo, std::size_t up, Iterator i) {
-    auto j = m.upper_bound(up);
-    // up < j->first
-    auto cu = std::prev(j)->second;
-    // cu < j->first
-    if (cu > up)
-      up = cu;
-    m.erase(i, j);
-    // merge [i .. j) with [i->first, max(up, cu)]
-    m.insert({lo, up});
-  }
-
-  MAP m{};
-};
-
-} // namespace Fortran::lower
-
-#endif // FORTRAN_LOWER_INTERVALSET_H