constinit not working as expected in Visual Studio 2019 Update 16.10

[maxgolov]

Fixes #8688

Visual Studio 2019 Update 16.10 introduced support for constinit for the very first time on May 25th. Unfortunately the way how protobuf uses constinit does not work with Visual Studio msvc compiler. This is a temporary solution to unblock those who updated to latest release of Visual Studio 2019 compiler.

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[maxgolov]

@googlebot I signed it!

[maxgolov]

I don't have permission to add label release notes: no. This change is for c++, the scope of it is too small for inclusion in release notes.

[fowles]

Thanks for the patch! will merge after CI finishes