[llvm] [ADT] Make SmallVector satisfy is_nothrow_move_constructible (PR #147684)

[Jay Foad via llvm-commits]

jayfoad wrote:

There is no measurable impact on compile time, but a very small space saving: https://llvm-compile-time-tracker.com/compare.php?from=5987f1ee5cc59a05961156c04010ab0f3c857628&to=b11bd294d9bc57210ab575c1a326d26a0e836464&stat=instructions:u

https://github.com/llvm/llvm-project/pull/147684