[llvm] Allow SMLoc to be used in constexpr context

Since `StringRef::empty` can be used in constexpr context, it seems reasonable that `SMLoc::isValid` should be too.  The default-ctor and `operator==` are made constexpr for consistency.

In particular, the `constexpr` keyword is needed for silencing warnings on Windows (whereas Linux allows constexpr usage without the keyword).

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D154741