Pull in some refactoring and a fix for SR-2242.
[Diagnostics] Refactor argument matching when dealing with a single c…
(cherry picked from commit f0078d6)
[Diagnostics] Refactor argument source range retrieval
(cherry picked from commit 2dfc8b6)
[Diagnostics] SR-2242: Fix diagnostic when argument label is omitted
(cherry picked from commit 054e3e4)
@swift-ci Please test OS X.
@DougGregor Can you review for swift-3.0-branch?
This looks fantastic!