stdlib: Workaround for IntervalType.overlaps to avoid a SIL parser error.

The parse_stdlib tests choke on the constraints that were there previously,
so weaken the API to only work on intervals of the same type for now.

rdar://problem/22126470

Swift SVN r30977

stdlib/public/core/Interval.swift.gyb

@@ -158,9 +158,7 @@ extension ClosedInterval {
 
 extension IntervalType {
   /// Returns `true` if `lhs` and `rhs` have a non-empty intersection.
-  public func overlaps<
-    I: IntervalType where I.Bound == Bound
-  >(other: I) -> Bool {
+  public func overlaps(other: Self) -> Bool {
     return contains(other.start) || other.contains(start)
   }
 }