[llvm] SmallVector: add missing constexpr annotations (NFC) (PR #98296)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

So, I'm currently building a 2-d matrix data structure, leveraging the SmallVector infrastructure. The issue with these not being marked constexpr is that nothing that uses them can be marked constexpr either: Windows fails the build on a constexpr expression, if any of the dependent expressions aren't explicitly marked constexpr.

https://github.com/llvm/llvm-project/pull/98296