borrow possibly useful ideas from swift's TypeCheckSwitchStmt.cpp file

[nishanths]

See if we can borrow any useful ideas related to exhaustiveness checking.

  /// 1. An algorithm for computing the exhaustiveness of a switch statement
  ///    using an algebra of spaces based on Fengyun Liu's
  ///    "A Generic Algorithm for Checking Exhaustivity of Pattern Matching".
  /// 2. An algorithm for computing warnings for pattern matching based on
  ///    Luc Maranget's "Warnings for pattern matching".
  ///
  /// The main algorithm centers around the computation of the difference and
  /// the containment of the "Spaces" given in each case, which reduces the
  /// definition of exhaustiveness to checking if the difference of the space
  /// 'S' of the user's written patterns and the space 'T' of the pattern
  /// condition is empty.

https://github.com/apple/swift/blob/ea142dba029b9c14bd778b04daca646319d53c4e/lib/Sema/TypeCheckSwitchStmt.cpp