Add test for deserializing opaque types across modules

Author: jckarter

test/Serialization/Inputs/OpaqueCrossFileB.swift

@@ -0,0 +1,9 @@
+public protocol Foo {}
+
+public struct FooImpl: Foo {
+  public init() {}
+}
+
+public func anyFoo() -> __opaque Foo {
+  return FooImpl()
+}

test/Serialization/opaque_cross_file.swift

@@ -0,0 +1,8 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module-path %t/OpaqueCrossFileB.swiftmodule -module-name OpaqueCrossFileB %S/Inputs/OpaqueCrossFileB.swift
+// RUN: %target-swift-frontend -I %t -emit-ir -verify %s
+
+import OpaqueCrossFileB
+
+dump(anyFoo())
+