swift234

fix <rdar://25178926> QoI: Warn about cases where switch statement "ignores" where clause

It is a common point of confusion that code like:

      switch value {
      case .Foo, .Bar where someNumber != 100:

Only applies the where clause to the second pattern, not every pattern in the case.

Resolve this by warning about the ambiguity, providing two notes (with fixits) that
resolve the issue in different ways:

t.swift:25:17: warning: 'where' only applies to the second pattern match in this case
case .Foo, .Bar where someNumber != 100:
           ~~~~ ^     ~~~~~~~~~~~~~~~~~
t.swift:25:12: note: disambiguate by adding a line break between them if this is desired
case .Foo, .Bar where someNumber != 100:
           ^
t.swift:25:6: note: duplicate the 'where' on both patterns to check both patterns
case .Foo, .Bar where someNumber != 100:
     ^~~~
          where someNumber != 100