<yvals_core.h>: Update _MSVC_STL_UPDATE to December 2019

[StephanTLavavej]

Explanation on our wiki: Macro _MSVC_STL_UPDATE

Code:

STL/stl/inc/yvals_core.h

Line 428 in 1e8b8d4

#define _MSVC_STL_UPDATE 201911L

Previous update: #247

This issue is intended for a new contributor (especially one new to GitHub) to get started with the simplest possible change.