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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I've gone through https://en.cppreference.com/w/cpp/language/constexpr, and there doesn't seem to be a relationship between constexpr functions and constexpr constructors, but maybe I'm missing something?

If you know the exact size, why not use std::array? 


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