Test:Unknown properties are assignable to Object in union

Add test and accept baselines

tests/baselines/reference/unknownPropertiesAreAssignableToObjectUnion.js

@@ -0,0 +1,8 @@
+//// [unknownPropertiesAreAssignableToObjectUnion.ts]
+const x: Object | string = { x: 0 };
+const y: Object | undefined = { x: 0 };
+
+
+//// [unknownPropertiesAreAssignableToObjectUnion.js]
+var x = { x: 0 };
+var y = { x: 0 };

tests/baselines/reference/unknownPropertiesAreAssignableToObjectUnion.symbols

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+=== tests/cases/compiler/unknownPropertiesAreAssignableToObjectUnion.ts ===
+const x: Object | string = { x: 0 };
+>x : Symbol(x, Decl(unknownPropertiesAreAssignableToObjectUnion.ts, 0, 5))
+>Object : Symbol(Object, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>x : Symbol(x, Decl(unknownPropertiesAreAssignableToObjectUnion.ts, 0, 28))
+
+const y: Object | undefined = { x: 0 };
+>y : Symbol(y, Decl(unknownPropertiesAreAssignableToObjectUnion.ts, 1, 5))
+>Object : Symbol(Object, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>x : Symbol(x, Decl(unknownPropertiesAreAssignableToObjectUnion.ts, 1, 31))
+

tests/baselines/reference/unknownPropertiesAreAssignableToObjectUnion.types

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+=== tests/cases/compiler/unknownPropertiesAreAssignableToObjectUnion.ts ===
+const x: Object | string = { x: 0 };
+>x : string | Object
+>Object : Object
+>{ x: 0 } : { x: number; }
+>x : number
+>0 : 0
+
+const y: Object | undefined = { x: 0 };
+>y : Object | undefined
+>Object : Object
+>{ x: 0 } : { x: number; }
+>x : number
+>0 : 0
+

tests/cases/compiler/unknownPropertiesAreAssignableToObjectUnion.ts

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+// @strictNullChecks: true
+const x: Object | string = { x: 0 };
+const y: Object | undefined = { x: 0 };