Improve warning suppressions for header units

[StephanTLavavej]

This is a perma-workaround for DevCom-10097021 / VSO-1581774 "C++ STL header <format> causes C4296 warning that need to be suppressed every time". @cdacamar is investigating the underlying compiler bug, where #pragmas within functions aren't properly stored within an IFC. In the meantime, if we consistently push/disable/pop around a function, we'll avoid this problem.

Most of our push/disable/pops are already outside functions, so only a couple of occurrences need to be changed:

• <format>: Move push/disable/pop outside of _Write_integral.
• <thread>: Move push/disable/pop outside _Start.
  • Also update comment to exactly match the printed warning.

And one more occurrence can avoid pragmas entirely (I verified that this continues to suppress the warning):

• <regex>: Use _Analysis_assume_ instead of push/disable/pop.

Next, this isn't being directly motivated by user bug reports, but I'm nervous about the occasional use of #pragma warning(suppress : NUM) in the headers. This seems likely to have similar problems with header units, and @AnjuDel recently encountered apparently squirrelly behavior of suppress in ATL/MFC. I think we should consistently push/disable/pop outside functions, which is a simple convention (and not too verbose):

• <ranges>: Replace 2x suppress with push/disable/pop outside of iota_view::size.
• <span>: Replace suppress with push/disable/pop outside size_bytes, front, back.
• <xstring>: Replace suppress with push/disable/pop outside of resize_and_overwrite.
  • Update comment due to the changed context.
• <atomic>: Replace suppress with push/disable/pop outside of _Atomic_thread_fence.
• <format>: Replace suppress with push/disable/pop outside of _Format_arg_store::_Store_impl.

Finally, one kind of suppression can be eliminated entirely, due to recent /analyze improvements:

• We don't need to suppress C6326 with if constexpr.

Click to expand example:

D:\GitHub\STL\out\build\x64>type woof.cpp

template <typename T>
int func() {
    constexpr auto N = sizeof(T);
#ifdef USE_IF_CONSTEXPR
    if constexpr (N < 4) {
#else
    if (N < 4) {
#endif
        return 11;
    } else {
        return 22;
    }
}

int main() {
    (void) func<short>();
    (void) func<double>();
}

D:\GitHub\STL\out\build\x64>cl /EHsc /nologo /W4 /analyze /std:c++latest /MTd /Od woof.cpp
woof.cpp
woof.cpp(7): warning C4127: conditional expression is constant
woof.cpp(2): note: consider using 'if constexpr' statement instead
woof.cpp(16): note: see reference to function template instantiation 'int func<short>(void)' being compiled
D:\GitHub\STL\out\build\x64\woof.cpp(7) : warning C6326: Potential comparison of a constant with another constant.
D:\GitHub\STL\out\build\x64\woof.cpp(7) : warning C6326: Potential comparison of a constant with another constant.

D:\GitHub\STL\out\build\x64>cl /EHsc /nologo /W4 /analyze /std:c++latest /MTd /Od /DUSE_IF_CONSTEXPR woof.cpp
woof.cpp

D:\GitHub\STL\out\build\x64>

Verified this change:

D:\GitHub\STL\out\build\x64>type meow.cpp

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

void print(const vector<int>& v) {
    for (const auto& e : v) {
        cout << e << ", ";
    }
    cout << "\n";
}

int main() {
    vector<int> result{-1, -1, -1, -1};

    vector<int> v{11, 22, 33, 44};
    print(v);
    reverse(v.begin(), v.end());
    print(v);
    ranges::reverse(v);
    print(v);

    print(result);
    reverse_copy(v.begin(), v.end(), result.begin());
    print(result);
    ranges::fill(result, -1);
    print(result);
    ranges::reverse_copy(v, result.begin());
    print(result);
}

D:\GitHub\STL\out\build\x64>cl /EHsc /nologo /W4 /analyze /std:c++latest /MTd /Od /I D:\GitHub\STL\stl\inc meow.cpp && meow
meow.cpp
11, 22, 33, 44,
44, 33, 22, 11,
11, 22, 33, 44,
-1, -1, -1, -1,
44, 33, 22, 11,
-1, -1, -1, -1,
44, 33, 22, 11,

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.