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

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar requested changes to this pull request.

> For instance, a constexpr computation involving SmallVector::size() cannot be marked constexpr. Not marking it as constexpr will not give us an inlining-guarantee, for instance. Another example of a serious issue is that we can't use a compile-time assertion on a non-constexpr expression: we'd have to use a runtime assertion instead.

Isn't a constexpr constructor a prerequisite for using these in the constexpr context anyway?

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