[Sema] NFC: Move FreeTypeVariableBinding to constraint system header

Author: xedin

lib/Sema/ConstraintSystem.h

@@ -106,6 +106,17 @@ enum ContextualTypePurpose {
   CTP_CannotFail,       ///< Conversion can never fail. abort() if it does.
 };
 
+/// Specify how we handle the binding of underconstrained (free) type variables
+/// within a solution to a constraint system.
+enum class FreeTypeVariableBinding {
+  /// Disallow any binding of such free type variables.
+  Disallow,
+  /// Allow the free type variables to persist in the solution.
+  Allow,
+  /// Bind the type variables to UnresolvedType to represent the ambiguity.
+  UnresolvedType
+};
+
 namespace constraints {
 
 /// Describes the algorithm to use for trailing closure matching.

lib/Sema/TypeChecker.h

@@ -181,17 +181,6 @@ enum class Comparison {
   Worse
 };
 
-/// Specify how we handle the binding of underconstrained (free) type variables
-/// within a solution to a constraint system.
-enum class FreeTypeVariableBinding {
-  /// Disallow any binding of such free type variables.
-  Disallow,
-  /// Allow the free type variables to persist in the solution.
-  Allow,
-  /// Bind the type variables to UnresolvedType to represent the ambiguity.
-  UnresolvedType
-};
-
 /// A conditional conformance that implied some other requirements. That is, \c
 /// ConformingType conforming to \c Protocol may have required additional
 /// requirements to be satisfied.