[llvm] [Triple] Add "swift" as a vendor. (PR #85094)

[Alastair Houghton via llvm-commits]

al45tair wrote:

> Changing rtlib/unwind/stdlib can be done with a configuration file as your PR demonstrates.

Not in the Swift compiler it can’t, because Swift doesn’t have config files, which is why the triple change is necessary.

Again, the motivation here is to avoid setting everything up in a way that causes problems for “stock” musl distributions later on.

https://github.com/llvm/llvm-project/pull/85094