[embedded] Apply shared linkage to metadata of public types in embedded with existential mode

We generate public metadata lazily which implies it could be emitted into a
different module. If we emit metadata for a public type into a module
other than its "home module" apply "shared" linkage.

test/embedded/lazy_metadata.swift

@@ -0,0 +1,109 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -wmo -module-name A -emit-irgen -o %t/A1.ll %t/A.swift -enable-experimental-feature Embedded -enable-experimental-feature EmbeddedExistentials -parse-as-library
+// RUN: %FileCheck --check-prefix=A1 %s < %t/A1.ll
+
+// RUN: %target-swift-frontend -wmo -module-name A -num-threads 1 -emit-ir -o %t/A2.ll -o %t/B2.ll %t/A.swift %t/B.swift -enable-experimental-feature Embedded -enable-experimental-feature EmbeddedExistentials -parse-as-library
+// RUN: %FileCheck --check-prefix=A2 %s < %t/A2.ll
+// RUN: %FileCheck --check-prefix=B2 %s < %t/B2.ll
+
+// RUN: %target-swift-frontend  -emit-module -emit-module-path %t/A.swiftmodule -wmo -module-name A -emit-ir -o %t/A3.ll  %t/A.swift %t/B.swift -enable-experimental-feature Embedded -enable-experimental-feature EmbeddedExistentials -parse-as-library
+// RUN: %FileCheck --check-prefix=A3 %s < %t/A3.ll
+
+// RUN: %target-swift-frontend -wmo -I %t -module-name C -emit-irgen -o %t/C4.ll  %t/C.swift -enable-experimental-feature Embedded -enable-experimental-feature EmbeddedExistentials -parse-as-library
+// RUN: %FileCheck --check-prefix=C4 %s < %t/C4.ll
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: optimized_stdlib
+// REQUIRES: swift_feature_Embedded
+// REQUIRES: swift_feature_EmbeddedExistentials
+
+
+//--- A.swift
+public class SomeClass {
+}
+
+public struct SomeStruct {
+  var x = 1
+  var y = 2
+}
+
+public enum SomeEnum {
+   case a(Int)
+   case b(Float)
+}
+
+//--- B.swift
+public func getSomeClass() ->  SomeClass {
+  return SomeClass()
+}
+
+public func getSomeStructAsAny() -> Any {
+  return SomeStruct()
+}
+
+public func getSomeStruct() -> SomeStruct {
+  return SomeStruct()
+}
+
+public func getSomeEnumAsAny() -> Any {
+  return SomeEnum.a(1)
+}
+
+public func getSomeEnum()-> SomeEnum {
+  return SomeEnum.b(2.0)
+}
+
+//--- C.swift
+import A
+public func useMetadata() -> Any {
+  return getSomeClass()
+}
+
+public func useMetadata2() -> Any {
+  return getSomeStruct()
+}
+
+public func useMetadata3() -> Any {
+  return getSomeEnum()
+}
+
+// Test no reference of metadata.
+// A1-NOT: CMf
+
+// Test reference of metadata. Because we are the defining module metadata is
+// visible outside of the image per current policy.
+// In multiple llvm modules per SIL module mode "Private" SIL linkage becomes
+// hidden linkonce_odr.
+// A2: @"$e1A8SomeEnumOMf" = linkonce_odr hidden constant
+// A2: @"$e1A10SomeStructVMf" = linkonce_odr hidden constant
+// A2: @"$e1A9SomeClassCMf" = linkonce_odr hidden global
+
+// A2: @"$e1A8SomeEnumON" = alias{{.*}}e1A8SomeEnumOMf
+// A2: @"$e1A10SomeStructVN" = alias{{.*}}e1A10SomeStructVMf
+// A2: @"$e1A9SomeClassCN" = alias{{.*}}e1A9SomeClassCMf
+
+// B2: @"$e1A9SomeClassCN" = {{.*}}external{{.*}} global
+// B2: @"$e1A10SomeStructVN" = {{.*}}external{{.*}} global
+// B2: @"$e1A8SomeEnumON" = {{.*}}external{{.*}} global
+// B2: call {{.*}} @"$e1A9SomeClassCACycfC"(ptr swiftself @"$e1A9SomeClassCN")
+// B2: store{{.*}}e1A10SomeStructVN
+// B2: store{{.*}}e1A8SomeEnumON
+
+// Test reference of metadata. Because we are the defining module metadata is
+// visible outside of the image per current policy.
+// In single llvm module per SIL module "Private" SIL linkage makes more
+// intuitive sense.
+// A3: @"$e1A8SomeEnumOMf" = internal constant
+// A3: @"$e1A10SomeStructVMf" = internal constant
+// A3: @"$e1A9SomeClassCMf" = internal global
+// A3: @"$e1A9SomeClassCN" = alias{{.*}}e1A9SomeClassCMf
+// A3: call {{.*}} @"$e1A9SomeClassCACycfC"({{.*}}getelementptr{{.*}}@"$e1A9SomeClassCMf"
+
+
+// Test "external" reference of metadata (defines metadata from another module
+// in current module as linkonce).
+// C4: @"$e1A8SomeEnumOMf" = linkonce_odr hidden constant
+// C4: @"$e1A10SomeStructVMf" = linkonce_odr hidden constant
+// C4: @"$e1A9SomeClassCMf" = linkonce_odr hidden global