[llvm] [ADT] Introduce Casting function objects (PR #165803)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

> > The tests can be simplified by casting individual elements instead of combining with map_range.
> 
> True, but I wanted to exercise an example of the targeted use case. WDYT?

We could have that as an extra test case for documentation etc., but otherwise I think it mainly adds complexity we don't need.

https://github.com/llvm/llvm-project/pull/165803