Re-commit r363191 "[MS] Pretend constexpr variable template specializations are inline"

While the next Visual Studio update (16.3) will fix this issue, that hasn't
shipped yet. Until then Clang wouldn't work with MSVC's headers which seems
unfortunate. Let's keep this in until VS 16.3 ships. (See also PR42843.)

> Fixes link errors with clang and the latest Visual C++ 14.21.27702
> headers, which was reported as PR42027.
>
> I chose to intentionally make these things linkonce_odr, i.e.
> discardable, so that we don't emit definitions of these things in every
> translation unit that includes STL headers.
>
> Note that this is *not* what MSVC does: MSVC has not yet implemented C++
> DR2387, so they emit fully specialized constexpr variable templates with
> static / internal linkage.
>
> Reviewers: rsmith
>
> Differential Revision: https://reviews.llvm.org/D63175

llvm-svn: 370850

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