[MS] Consder constexpr globals to be inline, as in C++17

Summary:
Microsoft seems to do this regardless of the language mode, so we must
also do it in order to be ABI compatible.

Fixes PR36125

Reviewers: thakis

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D47956

llvm-svn: 371642