[flang] Fix warning from MSVC build

[klausler]

Summary

Expand common::BitSet from 64 to 128 maximum elements.

[klausler]

MSVC still complained with a bogus warning; try again.

[vdonaldson]

Simpler is better if it works.

[klausler]

Take 3.

flang/include/flang/Common/constexpr-bitset.h

 //===-- include/flang/Common/constexpr-bitset.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_COMMON_CONSTEXPR_BITSET_H_
 #define FORTRAN_COMMON_CONSTEXPR_BITSET_H_
 
 // Implements a replacement for std::bitset<> that is suitable for use
-// in constexpr expressions.  Limited to elements in [0..63].
+// in constexpr expressions.  Limited to elements in [0..127].
 
 #include "bit-population-count.h"
+#include "uint128.h"
 #include <cstddef>
 #include <cstdint>
 #include <initializer_list>
 #include <optional>
 #include <type_traits>
 
 namespace Fortran::common {
 
 template <int BITS> class BitSet {
-  static_assert(BITS > 0 && BITS <= 64);
-  static constexpr bool partialWord{BITS != 32 && BITS != 64};
-  using Word = std::conditional_t<(BITS > 32), std::uint64_t, std::uint32_t>;
+  static_assert(BITS > 0 && BITS <= 128);
+  using Word = HostUnsignedIntType<(BITS <= 32 ? 32 : BITS)>;
   static constexpr Word allBits{
-      partialWord ? (static_cast<Word>(1) << BITS) - 1 : ~static_cast<Word>(0)};
+      ~static_cast<Word>(0) >> (8 * sizeof(Word) - BITS)};
 
   constexpr BitSet(Word b) : bits_{b} {}
 
 public:
   constexpr BitSet() {}
   constexpr BitSet(const std::initializer_list<int> &xs) {
     for (auto x : xs) {
       set(x);