<exception>: std::exception::what has wrong signature

Describe the bug
std::exception::what is not noexcept.
See also #882 - should probably be fixed together. #808 is also in similar area

Command-line test case

d:\Temp2>type repro.cpp
#include <cstdio>
#include <exception>

int main()
{
    auto b = noexcept(std::exception{}.what());
    std::printf("%d\n", (int)b);
}


d:\Temp2>cl /EHsc /permissive- /std:c++latest /Zc:__cplusplus /Zc:externConstexpr repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29009.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

/std:c++latest is provided as a preview of language features from the latest C++
working draft, and we're eager to hear about bugs and suggestions for improvements.
However, note that these features are provided as-is without support, and subject
to changes or removal as the working draft evolves. See
https://go.microsoft.com/fwlink/?linkid=2045807 for details.

repro.cpp
Microsoft (R) Incremental Linker Version 14.27.29009.1
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:repro.exe
repro.obj

d:\Temp2>.\repro.exe
0

Expected behavior
Should print 1 as ctor is noexcept.

STL version

Microsoft Visual Studio Professional 2019 Preview
Version 16.7.0 Preview 3.1

Additional context
This item is also tracked on Developer Community as DevCom-785027 and by Microsoft-internal VSO-1006425 / AB#1006425.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

Start with the command-line reproduction in repro.cpp and confirm that noexcept(std::exception{}.what()) currently prints 0. Read the related issues #882 and #808, then check the vNext branch and the binary-compatibility guidance in #169. Done means the test prints 1 without violating the stated compatibility constraints.