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

[Alastair Houghton via llvm-commits]

al45tair wrote:

@MaskRay Yes, however I _do_ need special behaviour in the Swift compiler, which uses the LLVM `Triple` class, hence I do actually need to add this here.

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