[MS] Make constexpr static data members implicitly inline

This handles them exactly the same way that we handle const integral
static data members with inline definitions, which is what MSVC does.

As a follow-up, now that we have a way to mark variables inline in the
AST, we should consider marking them implicitly inline there instead of
only treating them as inline in CodeGen. Unfortunately, this breaks a
lot of dllimport test cases, so that is future work for now.

Fixes PR36125.

llvm-svn: 325375