Add missing box instruction in pattern match (#18922)

Fixes #18859

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_PatternSwitchStatement.cs

@@ -402,6 +402,7 @@ private void LowerConstantValueDecision(DecisionTree.ByValue byValue)
                     {
                         var loweredRight = kv.Key;
                         var loweredLeft = kv.Value;
+                        loweredRight = _factory.Convert(loweredLeft.Type, loweredRight);
                         addBindings.Add(_factory.ExpressionStatement(
                             _localRewriter.MakeStaticAssignmentOperator(
                                 _factory.Syntax, loweredLeft, loweredRight, RefKind.None, loweredLeft.Type, false)));

src/Compilers/CSharp/Portable/Lowering/SyntheticBoundNodeFactory.cs

@@ -1104,6 +1104,7 @@ public BoundExpression Convert(TypeSymbol type, BoundExpression arg)
         {
             HashSet<DiagnosticInfo> useSiteDiagnostics = null;
             Conversion c = Compilation.Conversions.ClassifyConversionFromExpression(arg, type, ref useSiteDiagnostics);
+            Debug.Assert(c.Exists);
             Debug.Assert(useSiteDiagnostics.IsNullOrEmpty());
 
             // If this happens, we should probably check if the method has ObsoleteAttribute.