Fix warning C4018: '<': signed/unsigned mismatch

Again, `(_Urng::max) () - (_Urng::min) ()` is `result_type - result_type`, subject to the usual arithmetic conversions.
When compared against `_Udiff _Bmask_local`, this can emit signed/unsigned mismatch warnings.

(This repro is x86-specific because MSVC emits warning C4018 at level 3 for `int < unsigned int`.
`int < unsigned long long` emits an off-by-default warning C4388 "signed/unsigned mismatch".
So even though the warning behavior is architecture-neutral,
the varying size of `_Udiff` causes the warning to be x86-specific.)

I'm avoiding extracting the `max - min` as a constant to avoid VSO-2580691 "`/analyze` emits bogus warning C6295 (Loop executes infinitely) for a loop that immediately finishes".

Author: StephanTLavavej

stl/inc/algorithm

@@ -6112,7 +6112,7 @@ private:
     static constexpr size_t _Calc_bits() {
         auto _Bits_local  = _Udiff_bits;
         auto _Bmask_local = static_cast<_Udiff>(-1);
-        for (; (_Urng::max) () - (_Urng::min) () < _Bmask_local; _Bmask_local >>= 1) {
+        for (; static_cast<_Udiff>((_Urng::max) () - (_Urng::min) ()) < _Bmask_local; _Bmask_local >>= 1) {
             --_Bits_local;
         }