[llvm] [ADT] Add convenience function `filter_to_vector` (PR #117460)

[David Blaikie via llvm-commits]

dwblaikie wrote:

I probably had similar feedback on previous *_to_vector - but I do wonder whether these are worthwhile/that much of a benefit compared to composing the two operations when needed?

https://github.com/llvm/llvm-project/pull/117460