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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> How would you ever call a non-static constexpr member in a constexpr context if you cannot obtain an object of the class in a constexpr context, because the constructor is not constexpr?
> 
> These things are nominally independent, but non-static constexpr members are useless without constexpr ctors.

I see. Thanks for the explanation.

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