[Constraint solver] Initial work on a less horrible solver.

This solver handles most of what the old solver does (enabling it by
default only causes 9 test failures), but is radically simpler and
already significantly faster (~50% slower than the old-old type
checker on swift.swift). There are some egregious hacks here to still
be eliminated (see FIXMEs), but it's a start.


Swift SVN r5272

include/swift/Basic/LangOptions.h

@@ -29,6 +29,10 @@ namespace swift {
     /// solver is the only type checker.
     bool UseConstraintSolver = true;
 
+    /// \brief Use the new constraint solver, as opposed to the old constraint
+    /// solver.
+    bool UseNewConstraintSolver = false;
+    
     /// \brief Whether we are debugging the constraint solver.
     ///
     /// This option enables verbose debugging output from the constraint
@@ -37,6 +41,7 @@ namespace swift {
 
     /// \brief Map NSString -> String in function parameters and results.
     bool NSStringIsString = false;
+
   };
 }