[MS] Pretend constexpr variable template specializations are inline

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: 363191