foo(_: 3) is equivalent to foo(3), so calling a function that has _ as
an argument label (func foo(`_`: 3)) still requires the _ to be
escaped. Before this patch, the compiler would suggest removing the `s,
even though that changes behaviour.
Fixes rdar://problem/31077797.
Allow all keywords except for parameter introducers (var/let/inout) to
be argument labels when declaring or calling a
function/initializer/subscript, e.g., this
func touchesMatching(phase: NSTouchPhase, `in` view: NSView?) -> Set<NSTouch>
can now be expressed as
func touchesMatching(phase: NSTouchPhase, in view: NSView?) -> Set<NSTouch>
and the call goes from
event.touchesMatching(phase, `in`: view)
to
event.touchesMatching(phase, in: view)
Fixes [SR-344](https://bugs.swift.org/browse/SR-344) /
rdar://problem/22415674.
