Fix nullable warnings

Author: RikkiGibson

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

@@ -617,7 +617,7 @@ public override BoundNode VisitTypeOfOperator(BoundTypeOfOperator node)
             var type = this.VisitType(node.Type);
 
             // Emit needs this helper
-            MethodSymbol getTypeFromHandle;
+            MethodSymbol? getTypeFromHandle;
             if (!TryGetWellKnownTypeMember(node.Syntax, WellKnownMember.System_Type__GetTypeFromHandle, out getTypeFromHandle))
             {
                 return new BoundTypeOfOperator(node.Syntax, sourceType, null, type, hasErrors: true);
@@ -634,7 +634,7 @@ public override BoundNode VisitRefTypeOperator(BoundRefTypeOperator node)
             var type = this.VisitType(node.Type);
 
             // Emit needs this helper
-            MethodSymbol getTypeFromHandle;
+            MethodSymbol? getTypeFromHandle;
             if (!TryGetWellKnownTypeMember(node.Syntax, WellKnownMember.System_Type__GetTypeFromHandle, out getTypeFromHandle))
             {
                 return new BoundRefTypeOperator(node.Syntax, operand, null, type, hasErrors: true);

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

@@ -106,7 +106,7 @@ private BoundExpression MakeUtf8Span(BoundExpression node, IReadOnlyList<byte>?
                                             BadExpression(node.Syntax, byteArray, ImmutableArray<BoundExpression>.Empty) :
                                             MakeUnderlyingArrayForUtf8Span(node.Syntax, byteArray, bytes, out length);
 
-            if (!TryGetWellKnownTypeMember<MethodSymbol>(node.Syntax, WellKnownMember.System_ReadOnlySpan_T__ctor_Array_Start_Length, out MethodSymbol ctor))
+            if (!TryGetWellKnownTypeMember<MethodSymbol>(node.Syntax, WellKnownMember.System_ReadOnlySpan_T__ctor_Array_Start_Length, out MethodSymbol? ctor))
             {
                 result = BadExpression(node.Syntax, node.Type, ImmutableArray<BoundExpression>.Empty);
             }

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

@@ -113,7 +113,7 @@ private BoundExpression RewriteWindowsRuntimeEventAssignmentOperator(SyntaxNode
             BoundExpression? clearCall = null;
             if (kind == EventAssignmentKind.Assignment)
             {
-                MethodSymbol clearMethod;
+                MethodSymbol? clearMethod;
                 if (TryGetWellKnownTypeMember(syntax, WellKnownMember.System_Runtime_InteropServices_WindowsRuntime_WindowsRuntimeMarshal__RemoveAllEventHandlers, out clearMethod))
                 {
                     clearCall = MakeCall(
@@ -154,7 +154,7 @@ private BoundExpression RewriteWindowsRuntimeEventAssignmentOperator(SyntaxNode
 
             BoundExpression marshalCall;
 
-            MethodSymbol marshalMethod;
+            MethodSymbol? marshalMethod;
             if (TryGetWellKnownTypeMember(syntax, helper, out marshalMethod))
             {
                 marshalMethod = marshalMethod.Construct(eventType);