It sounds like you want to do this b/c of 2. What is this? Bumping the cmake version is a big thing to do. What is this "uswift"? And what are you really trying to do.
uSwift is a "micro" Swift standard library created by @compnerd. As I discuss in the forum post here and the PR here, I'd like to use this to cross-compile some of the C++ interop tests.
While you're right that I mainly want to update the CMake version so that we can use uswift, the reason that uswift uses CMake 3.18 (and not an earlier version) is important, and a good reason for us, too, to update our CMake version. Especially as we have more Swift code in the compiler/build pipeline.