[Sema] Add test case for protcol requirement accessibility across files

Author: rintaro

test/attr/accessibility_multifile.swift

@@ -0,0 +1,19 @@
+// RUN: rm -rf %t && mkdir -p %t
+// RUN: %utils/split_file.py -o %t %s
+// RUN: %target-swift-frontend -parse %t/file1.swift -primary-file %t/file2.swift -verify
+
+// BEGIN file1.swift
+private protocol P  {
+  func privMethod()
+}
+public class C : P {
+  public init() {}
+  fileprivate func privMethod() {}
+}
+
+// BEGIN file2.swift
+extension C {
+  public func someFunc() {
+    privMethod() // expected-error {{use of unresolved identifier 'privMethod'}}
+  }
+}