[MS] Silence -Wextern-init on const selectany variables

In C, 'extern' is typically used to avoid tentative definitions when
declaring variables in headers, but adding an intializer makes it a
defintion. This is somewhat confusing, so GCC and Clang both warn on it.
In C++, 'extern' is often used to give implictly static 'const'
variables external linkage, so don't warn in that case. If selectany is
present, this might be header code intended for C and C++ inclusion, so
apply the C++ rules.

llvm-svn: 279116