[ConstraintSystem] NFC: Make SemaTest a friend of ConstraintSystem

This is necessary in order to have access to private members of
a `ConstraintSystem` for testing purposes, such as logic related
to potential binding computation.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -66,6 +66,12 @@ class SolutionApplicationTarget;
 
 } // end namespace constraints
 
+namespace unittest {
+
+class SemaTest;
+
+} // end namespace unittest
+
 // Forward declare some TypeChecker related functions
 // so they could be made friends of ConstraintSystem.
 namespace TypeChecker {
@@ -2018,6 +2024,8 @@ enum class SolutionApplicationToFunctionResult {
 class ConstraintSystem {
   ASTContext &Context;
 
+  friend class swift::unittest::SemaTest;
+
 public:
   DeclContext *DC;
   ConstraintSystemOptions Options;