Only foreign thunks need to be serialized, not foreign definitions

This eliminates a SIL verification error with `@c` functions, which
provide definitions for foreign entrypoints. We were serializing @c
definitions when we shouldn't be, which would cause problems down the
line if those @c definitions referenced something internal that they
shouldn't.

Author: DougGregor

lib/SIL/IR/SILDeclRef.cpp

@@ -1025,7 +1025,8 @@ SerializedKind_t SILDeclRef::getSerializedKind() const {
     // @objc thunks for top-level functions are serializable since they're
     // referenced from @convention(c) conversions inside inlinable
     // functions.
-    return IsSerialized;
+    if (isThunk())
+      return IsSerialized;
   }
 
   // Declarations imported from Clang modules are serialized if

test/Interop/Cxx/extern-c/decls.swift

@@ -6,4 +6,4 @@ func zerome(ptr: UnsafeMutablePointer<Int>) {
 }
 
 // Verify that the asmname is "memset", not a C++-mangled version
-// CHECK: sil [serialized] [asmname "memset"] [clang memset] @$sSo6memsetySvSgAB_s5Int32VSitFTo : $@convention(c) (Optional<UnsafeMutableRawPointer>, Int32, Int) -> Optional<UnsafeMutableRawPointer>
+// CHECK: sil [asmname "memset"] [clang memset] @$sSo6memsetySvSgAB_s5Int32VSitFTo : $@convention(c) (Optional<UnsafeMutableRawPointer>, Int32, Int) -> Optional<UnsafeMutableRawPointer>

test/Interop/Cxx/foreign-reference/singleton-silgen.swift

@@ -35,4 +35,4 @@ public func test() {
 
 // CHECK-LABEL: sil {{.*}}[asmname "{{.*}}test{{.*}}"] [clang DeletedSpecialMembers.test] @$sSo21DeletedSpecialMembersV4tests5Int32VyFTo : $@convention(cxx_method) (DeletedSpecialMembers) -> Int32
 
-// CHECK-LABEL: sil [serialized] [asmname "{{.*}}mutate{{.*}}"] [clang mutateIt] @$sSo8mutateItyySo21DeletedSpecialMembersVFTo : $@convention(c) (DeletedSpecialMembers) -> ()
+// CHECK-LABEL: sil [asmname "{{.*}}mutate{{.*}}"] [clang mutateIt] @$sSo8mutateItyySo21DeletedSpecialMembersVFTo : $@convention(c) (DeletedSpecialMembers) -> ()

test/SILGen/cdecl-official.swift

@@ -33,7 +33,7 @@ func requiresThunk() {
   acceptSwiftFunc(orange)
 }
 
-// CHECK-LABEL: sil [serialized] [asmname "cauliflower"] [ossa] @$s5cdecl8broccoliyS2iFTo : $@convention(c) (Int) -> Int {
+// CHECK-LABEL: sil [asmname "cauliflower"] [ossa] @$s5cdecl8broccoliyS2iFTo : $@convention(c) (Int) -> Int {
 // CHECK-NOT: apply
 // CHECK: return
 @c(cauliflower)

test/SILGen/extern_c.swift

@@ -10,7 +10,7 @@ func withCName(_ x: Int) -> Int
 @_extern(c, "take_c_func_ptr")
 func takeCFuncPtr(_ f: @convention(c) (Int) -> Int)
 
-// CHECK-DAG: sil [serialized] [asmname "public_visible"] @$s8extern_c16publicVisibilityyS2iFTo : $@convention(c) (Int) -> Int
+// CHECK-DAG: sil [asmname "public_visible"] @$s8extern_c16publicVisibilityyS2iFTo : $@convention(c) (Int) -> Int
 @_extern(c, "public_visible")
 public func publicVisibility(_ x: Int) -> Int
 

test/embedded/linkage/leaf_application.swift

@@ -26,9 +26,6 @@
 // Never referenced.
 // LIBRARY-IR-NOT: @"$es23_swiftEmptyArrayStorageSi_S3itvp" = linkonce_odr {{(protected |dllexport )?}}global
 
-// Note: referenced by swift_allocEmptyBox.
-// LIBRARY-IR: @"$es16_emptyBoxStorageSi_Sitvp" = linkonce_odr {{(protected |dllexport )?}}global
-
 // LIBRARY-IR-NOT: define {{.*}}@"$e7Library5helloSaySiGyF"()
 public func hello() -> [Int] {
   getArray()

test/embedded/serialization.swift

@@ -0,0 +1,32 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// Library module
+
+// SIL checking
+// RUN: %target-swift-frontend %t/Library.swift -parse-as-library -entry-point-function-name Library_main -enable-experimental-feature Embedded -enable-experimental-feature DeferredCodeGen -emit-sil -emit-module-path %t/Modules/Library.swiftmodule -o - | %FileCheck -check-prefix LIBRARY-SIL %s
+
+// RUN: %target-swift-frontend %t/Application.swift -I %t/Modules -parse-as-library -entry-point-function-name Application_main -enable-experimental-feature Embedded -emit-sil -o - | %FileCheck -check-prefix APPLICATION-SIL %s
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: swift_feature_Embedded
+// REQUIRES: swift_feature_DeferredCodeGen
+
+//--- Library.swift
+
+func internalFunc() { }
+
+// LIBRARY-SIL: sil [asmname "swift_dosomething"] @$e7Library17swift_dosomethingyyFTo : $@convention(c) () -> () {
+@c
+public func swift_dosomething() {
+  internalFunc()
+}
+
+//--- Application.swift
+import Library
+
+// APPLICATION-SIL-LABEL: sil @$e11Application4testyyF : $@convention(thin) () -> ()
+public func test() {
+  // CHECK: function_ref @$e7Library17swift_dosomethingyyFTo : $@convention(c) () -> ()
+  swift_dosomething()
+}