Add a test case for AST verification of loaded modules in WMO.

Prior to the following commit, the SILVerifier will assert
on this test case.

PR: 14250 <#14250>

commit deebe8b
Author: Andrew Trick <[email protected]>
Date:   Mon Jan 29 15:22:28 2018

    performTypeChecking: Defer verifyAllLoadedModules in WMO mode.

Author: atrick

validation-test/Sema/Inputs/rdar36801676.swift

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+import Cocoa
+extension Notification.Name {}

validation-test/Sema/Inputs/rdar36801676_empty.swift

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+// Empty swift source file.

validation-test/Sema/wmo_verify_loaded.swift

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+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %target-swift-frontend -swift-version 4 -emit-module -o %t/rdar36801676.swiftmodule %S/Inputs/rdar36801676.swift
+// RUN: %target-swift-frontend -swift-version 4 -emit-silgen -enable-objc-interop -I %t -emit-silgen %S/Inputs/rdar36801676_empty.swift %s | %FileCheck %s
+// REQUIRES: OS=macosx
+
+// If AST loaded module verification is run after type checking the empty source
+// file (rdar36801676_empty.swift), but before type checking this source file,
+// then the importer caches the declaration for PasteboardType's constructor
+// without synthesizing it's body. Eventually, the SILVerifier will raise a linkage error:
+// SIL verification failed: external declarations of SILFunctions with shared
+// visibility is not allowed: SingleFunction ||
+// !hasSharedVisibility(RefF->getLinkage()) || RefF->hasForeignBody()
+
+import Cocoa
+import rdar36801676
+
+let objCSynthesizedEnum = NSPasteboard.PasteboardType(rawValue: "MyPboardType")
+extension Notification.Name {}
+
+// NSPasteboardType.init(rawValue:)
+// - just make sure it has a body.
+// CHECK-LABEL: sil shared [transparent] [serializable] @$SSo16NSPasteboardTypea8rawValueABSS_tcfC : $@convention(method) (@owned String, @thin NSPasteboard.PasteboardType.Type) -> @owned NSPasteboard.PasteboardType {
+// CHECK: bb0(%0 : $String, %1 : $@thin NSPasteboard.PasteboardType.Type):
+// CHECK: return %{{.*}} : $NSPasteboard.PasteboardType
+// CHECK-LABEL: } // end sil function '$SSo16NSPasteboardTypea8rawValueABSS_tcfC'