Add a comment about linux-static.

"linux-static" refers to the new fully-statically-linked Linux support; the point here is that we want to distinguish that from just "Linux with musl", which is an entirely reasonable thing for us to support separately. rdar://123442522

Author: al45tair

Sources/SwiftDriver/Utilities/Triple+Platforms.swift

@@ -294,6 +294,8 @@ extension Triple {
  case .linux:
  switch environment {
  case .musl:
+ // The triple for linux-static is <arch>-swift-linux-musl, to distinguish
+ // it from a "normal" musl set-up (ala Alpine).
  if vendor == .swift {
  return "linux-static"
  }