[llvm] [ADT] Allow SmallVector move construction without move assignment (PR #219934)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke approved this pull request.

LGTM. (But please add the motivation to the commit message -- it should explain why a certain change is done.)

https://github.com/llvm/llvm-project/pull/219934