`<bitset>`: `operator[]` could be SAL annotated with `_In_range_(<, _Bits)`

[StephanTLavavej]

Following array's example:

STL/stl/inc/array

Line 533 in 2314e1a

_NODISCARD _CONSTEXPR17 reference operator[](_In_range_(<, _Size) size_type _Pos) noexcept /* strengthened */ {

Found while investigating STL Hardening (#5090). I'm filing a followup issue because libcxx has tests under std/utilities/template.bitset that will emit /analyze warnings for bitset<0>. if constexpr could avoid this, but they compile their tests in older Standard modes.