Strict optional properties (#43947)

* Introduce --strictOptionalProperties compiler switch

* Accept new baselines

* Removing missingType when printing back optional properties

* Accept new baselines

* Fix linting issue

* Use getNonMissingTypeOfSymbol in getTypeOfSymbolAtLocation

* Properly elaborate errors involving optional properties

* Accept new baselines

* Properly check optional properties in tuple types

* Accept new baselines

* Add missing tuple type check

* More permissive check of strict optional properties and index signatures

* Add tests

* Fix lint issues

* Accept new baselines

src/compiler/commandLineParser.ts

@@ -612,6 +612,15 @@ namespace ts {
             category: Diagnostics.Strict_Type_Checking_Options,
             description: Diagnostics.Enable_strict_checking_of_property_initialization_in_classes
         },
+        {
+            name: "strictOptionalProperties",
+            type: "boolean",
+            affectsSemanticDiagnostics: true,
+            strictFlag: true,
+            showInSimplifiedHelpView: true,
+            category: Diagnostics.Strict_Type_Checking_Options,
+            description: Diagnostics.Enable_strict_checking_of_optional_properties
+        },
         {
             name: "noImplicitThis",
             type: "boolean",