[AutoDiff] Add SynthesizedFileUnit TBDGen/serialization tests.

Upstream `SynthesizedFileUnit` failing tests from TF-1239.

Author: dan-zheng

test/AutoDiff/compiler_crashers_fixed/Inputs/tf1202-differentiability-witness-dead-function-elimination.swift

@@ -0,0 +1,9 @@
+import _Differentiation
+
+@inlinable
+@differentiable(where T: Differentiable)
+public func identity<T>(_ x: T) -> T { x }
+
+public func foo<T: Differentiable>(_ f: @differentiable (T) -> T = identity) -> T {
+  fatalError()
+}

test/AutoDiff/compiler_crashers_fixed/tf1202-differentiability-witness-dead-function-elimination.swift

@@ -0,0 +1,17 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -emit-module -module-name tf1202 -emit-module-path %t/tf1202.swiftmodule %S/Inputs/tf1202-differentiability-witness-dead-function-elimination.swift
+// RUN: %target-build-swift -I%t -emit-module -O %s
+
+// TF-1202: test bug where DeadFunctionElimination eliminated the
+// SILFunction for `func identity<T>` even though a differentiability witness for it
+// exists. This causes deserialization of this module to crash when
+// trying to deserialize the differentiability witness because it can't find
+// the original function `func identity<T>`.
+
+// TF-1239: Test `SynthesizedFileUnit` serialization.
+
+import tf1202
+
+func callit() -> Float {
+  return foo()
+}

test/AutoDiff/validation-test/Inputs/cross_module_differentiation_other.swift

@@ -0,0 +1,14 @@
+import _Differentiation
+
+@differentiable
+public func defaultArgument(_ x: Float) -> Float {
+  return x
+}
+
+@differentiable
+public func applyArgument(
+  _ x: Float,
+  _ f: @differentiable (Float) -> Float = defaultArgument
+) -> Float {
+  return f(x)
+}

test/AutoDiff/validation-test/cross_module_differentiation.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -working-directory %t -parse-as-library -emit-module -module-name cross_module_differentiation_other -emit-module-path %t/cross_module_differentiation_other.swiftmodule -emit-library -static %S/Inputs/cross_module_differentiation_other.swift
+// RUN: %target-build-swift -I%t -L%t %s -o %t/a.out -lcross_module_differentiation_other
+// RUN: %target-run %t/a.out
+// REQUIRES: executable_test
+
+// TF-1025: Test differentiability witness linkage for `PublicNonABI` original functions.
+// TF-1239: Test `SynthesizedFileUnit` TBDGen.
+
+import cross_module_differentiation_other
+import _Differentiation
+import StdlibUnittest
+
+var CrossModuleTests = TestSuite("E2ECrossModule")
+
+CrossModuleTests.test("differentiable function default argument") {
+  let actualGrad = gradient(at: 0) { applyArgument($0) }
+  let expectedGrad: Float = 1
+  expectEqual(actualGrad, expectedGrad)
+}
+
+CrossModuleTests.test("differentiable function specified default argument") {
+  let actualGrad = gradient(at: 0) { applyArgument($0, { $0 }) }
+  let expectedGrad: Float = 1
+  expectEqual(actualGrad, expectedGrad)
+}
+
+runAllTests()