Tests: Verify ExtensionImportVisibility works for @_exported imports.

Author: tshortli

test/NameLookup/Inputs/extensions_A.swift

@@ -7,3 +7,11 @@ public struct Y<T> { }
 extension Y: P where T: P { }
 
 public struct Z: P { }
+
+extension X {
+  public func XinA() { }
+}
+
+extension Y {
+  public func YinA() { }
+}

test/NameLookup/Inputs/extensions_C.swift

@@ -1,4 +1,4 @@
-import extensions_A
+@_exported import extensions_A
 import extensions_B
 
 extension X {

test/NameLookup/extensions_transitive.swift

@@ -4,10 +4,12 @@
 // RUN: %target-swift-frontend -emit-module -I %t -o %t %S/Inputs/extensions_C.swift
 // RUN: %target-swift-frontend -typecheck %s -I %t -verify -enable-experimental-feature ExtensionImportVisibility
 
-import extensions_A
 import extensions_C
 // expected-note 2{{add import of module 'extensions_B'}}{{1-1=import extensions_B\n}}
 func test(x: X, y: Y<Z>) {
+  x.XinA()
+  y.YinA()
+
   x.XinB() // expected-error{{instance method 'XinB()' is not available due to missing import of defining module 'extensions_B'}}
   y.YinB() // expected-error{{instance method 'YinB()' is not available due to missing import of defining module 'extensions_B'}}