<yvals_core.h>: Update _MSVC_STL_UPDATE to February 2020

Explanation on our wiki: [Macro `_MSVC_STL_UPDATE`](https://github.com/microsoft/STL/wiki/Macro-_MSVC_STL_UPDATE)

Code:
https://github.com/microsoft/STL/blob/10e92884618d3b816a4dd54ffe21c4979ec5ddd6/stl/inc/yvals_core.h#L439

Previous update: #409

This issue is intended for a new contributor (especially one new to GitHub) to get started with the simplest possible change.