[CodeCompletion] Forward declare types in constraints instead of importing ConstraintSystem.h

Importing ConstraintSystem.h caused some build failures on Windows while building LLDB (which incidentally is also importing IDETypeChecking.h). Forward declare them to work around the build failure.

Author: ahoppen

include/swift/Sema/IDETypeChecking.h

@@ -21,7 +21,6 @@
 
 #include "swift/AST/Identifier.h"
 #include "swift/Basic/SourceLoc.h"
-#include "swift/Sema/ConstraintSystem.h"
 #include <memory>
 #include <tuple>
 
@@ -47,6 +46,12 @@ namespace swift {
   class ValueDecl;
   struct PrintOptions;
 
+  namespace constraints {
+  class ConstraintSystem;
+  class Solution;
+  class SolutionApplicationTarget;
+  }
+
   /// Typecheck binding initializer at \p bindingIndex.
   void typeCheckPatternBinding(PatternBindingDecl *PBD, unsigned bindingIndex,
                                bool leaveClosureBodiesUnchecked);