[AutoDiff] @_alwaysEmitIntoClient registration tests (#29129)

Here are some negative tests demonstrating the problem in TF-1103.

Author: marcrasi

test/AutoDiff/downstream/Inputs/always_emit_into_client/MultiFileModule/file1.swift

@@ -0,0 +1,4 @@
+@_alwaysEmitIntoClient
+public func f(_ x: Float) -> Float {
+  x
+}

test/AutoDiff/downstream/Inputs/always_emit_into_client/MultiFileModule/file2.swift

@@ -0,0 +1,5 @@
+@derivative(of: f)
+@_alwaysEmitIntoClient
+public func df(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
+  (x, { 10 * $0 })
+}

test/AutoDiff/downstream/Inputs/always_emit_into_client/SingleFileModule/file.swift

@@ -0,0 +1,10 @@
+@_alwaysEmitIntoClient
+public func f(_ x: Float) -> Float {
+  x
+}
+
+@derivative(of: f)
+@_alwaysEmitIntoClient
+public func df(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
+  (x, { 10 * $0 })
+}

test/AutoDiff/downstream/always_emit_into_client_multi_file.swift

@@ -0,0 +1,21 @@
+// TODO(TF-1103): Fix this test so that there is not a linker error. Then, move this test to
+// cross_module_derivative_attr_e2e.swift.
+
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -Xfrontend -enable-experimental-cross-file-derivative-registration -parse-as-library -emit-module -module-name MultiFileModule -emit-module-path %t/MultiFileModule.swiftmodule -emit-library -o %t/%target-library-name(MultiFileModule) %S/Inputs/always_emit_into_client/MultiFileModule/file1.swift %S/Inputs/always_emit_into_client/MultiFileModule/file2.swift
+// RUN: not %target-build-swift -I%t -L%t %s -o %t/a.out -lm -lMultiFileModule 2>&1 | %FileCheck %s
+
+import StdlibUnittest
+
+import MultiFileModule
+
+var AlwaysEmitIntoClientTests = TestSuite("AlwaysEmitIntoClient")
+
+AlwaysEmitIntoClientTests.test("registration") {
+  expectEqual(10, gradient(at: 0, in: f))
+}
+
+runAllTests()
+
+// CHECK: {{[Uu]}}ndefined
+// CHECK: AD__$s15MultiFileModule1fyS2fF_PSRS

test/AutoDiff/downstream/always_emit_into_client_single_file.swift

@@ -0,0 +1,21 @@
+// TODO(TF-1103): Fix this test so that there is not a linker error. Then, move this test to
+// cross_module_derivative_attr_e2e.swift.
+
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -Xfrontend -enable-experimental-cross-file-derivative-registration -parse-as-library -emit-module -module-name SingleFileModule -emit-module-path %t/SingleFileModule.swiftmodule -emit-library -o %t/%target-library-name(SingleFileModule) %S/Inputs/always_emit_into_client/SingleFileModule/file.swift
+// RUN: not %target-build-swift -I%t -L%t %s -o %t/a.out -lm -lSingleFileModule 2>&1 | %FileCheck %s
+
+import StdlibUnittest
+
+import SingleFileModule
+
+var AlwaysEmitIntoClientTests = TestSuite("AlwaysEmitIntoClient")
+
+AlwaysEmitIntoClientTests.test("registration") {
+  expectEqual(10, gradient(at: 0, in: f))
+}
+
+runAllTests()
+
+// CHECK: {{[Uu]}}ndefined
+// CHECK: AD__$s16SingleFileModule1fyS2fF_PSRS