[embedded] Relax assert in SILLinkerVisitor::maybeAddFunctionToWorklist to allow external forward-declarations even in regular Swift, add tests

test/embedded/modules-extern.swift

@@ -0,0 +1,33 @@
+// RUN: %empty-directory(%t)
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// RUN: %target-swift-frontend -emit-module -o %t/MyModule.swiftmodule %t/MyModule.swift -enable-experimental-feature Embedded -parse-as-library
+// RUN: %target-swift-frontend -c -I %t %t/Main.swift -enable-experimental-feature Embedded -o %t/a.o
+
+// REQUIRES: swift_in_compiler
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+// BEGIN MyModule.swift
+
+@_extern(c)
+func some_c_api()
+
+@_transparent
+public func publicFuncInAModule() {
+  internalFuncInAModule()
+}
+
+@usableFromInline
+internal func internalFuncInAModule() {
+  some_c_api()
+}
+
+// BEGIN Main.swift
+
+import MyModule
+
+@_extern(c)
+func some_c_api()
+
+some_c_api()
+publicFuncInAModule()