[Triple] Add "swift" as a vendor.

[al45tair]

We want this so we can uniquely identify our fully-static Linux target using a triple (since we wish to make decisions about it in the compiler driver that wouldn't be appropriate for an "unknown" triple).

rdar://123436421

[al45tair]

This is a cherry pick of #8256.

[bnbarham]

This branch is effectively dead, cherry-picked to the current stable branch (https://github.com/apple/llvm-project/pull/8753). I'll also deal with cherry-picking this to next (I'll bundle up all my current stable/20240408 changes when we create yet another stable branch).