[Decl] Add tests for invalid extension and multi file access of init accessors

Author: li3zhen1

test/decl/var/Inputs/imported_init_accessor.swift

@@ -0,0 +1,48 @@
+struct S {
+  var _x: Int
+  public var xImmutablePublic: Int {
+    @storageRestrictions(initializes: _x)
+    init {
+      _x = initialValue
+    }
+    get {
+      return _x
+    }
+  }
+  
+  public var xMutablePublic: Int {
+    @storageRestrictions(initializes: _x)
+    init {
+      _x = initialValue
+    }
+    get {
+      return _x
+    }
+    set {
+      _x = newValue
+    }
+  }
+  
+  internal var xImmutableInternal: Int {
+    @storageRestrictions(initializes: _x)
+    init {
+      _x = initialValue
+    }
+    get {
+      return _x
+    }
+  }
+  
+  internal var xMutableInternal: Int {
+    @storageRestrictions(initializes: _x)
+    init {
+      _x = initialValue
+    }
+    get {
+      return _x
+    }
+    set {
+      _x = newValue
+    }
+  }
+}

test/decl/var/init_accessors.swift

@@ -690,3 +690,19 @@ do {
   _ = Test2(q: "some question") // Ok `a` is default initialized to `nil`
   _ = Test2(q: "ultimate question", a: 42)
 }
+
+struct Test2ForExtension {
+  var _x: Int
+}
+
+extension Test2ForExtension {
+  var extendedX: Int {
+    @storageRestrictions(initializes: _x)
+    init {
+    // expected-error@-1 {{init accessors could only be declared in the primary declaration}}
+      self._x = newValue
+    }
+    get { _x }
+    set { _x = newValue }
+  }
+}

test/decl/var/init_accessors_multi_file.swift

@@ -0,0 +1,13 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -verify -O -primary-file %s %S/Inputs/imported_init_accessor.swift -c -o %t/init_accessors_multi_file.o
+
+extension S {
+  init(extendedX: Int) {
+    self.xImmutablePublic = extendedX
+  }
+}
+
+func test() {
+  let s = S(extendedX: 42)
+  _ = s.xImmutablePublic
+}