[Type checker] Use typeCheckCondition() rather than fake it.

Type checking an expression pattern had a hack to work around some diagnostics
issues with conditions. These issues have been addressed by the new
diagnostic infrastructure, so remove this unnecessary use of
ExprTypeCheckListener and check the expression pattern condition using…
typeCheckCondition().

Author: DougGregor

lib/Sema/TypeCheckConstraints.cpp

@@ -3209,30 +3209,7 @@ bool TypeChecker::typeCheckExprPattern(ExprPattern *EP, DeclContext *DC,
                                              /*Implicit=*/true);
 
   // Check the expression as a condition.
-  //
-  // TODO: Type-check of `~=` operator can't (yet) use `typeCheckCondition`
-  // because that utilizes contextual type which interferes with diagnostics.
-  // We don't yet have a full access to pattern-matching context in
-  // constraint system, which is required to enable these situations
-  // to be properly diagnosed.
-  struct ConditionListener : public ExprTypeCheckListener {
-    // Add the appropriate Boolean constraint.
-    bool builtConstraints(ConstraintSystem &cs, Expr *expr) override {
-      // Otherwise, the result must be convertible to Bool.
-      auto boolDecl = cs.getASTContext().getBoolDecl();
-      if (!boolDecl)
-        return true;
-
-      // Condition must convert to Bool.
-      cs.addConstraint(ConstraintKind::Conversion, cs.getType(expr),
-                       boolDecl->getDeclaredType(),
-                       cs.getConstraintLocator(expr));
-      return false;
-    }
-  };
-
-  ConditionListener listener;
-  bool hadError = !typeCheckExpression(matchCall, DC, &listener);
+  bool hadError = typeCheckCondition(matchCall, DC);
   // Save the type-checked expression in the pattern.
   EP->setMatchExpr(matchCall);
   // Set the type on the pattern.