[AutoDiff] Add @_exported import _Differentiation test. (swiftlang#30769)

Verify that importing a module with `@_exported import _Differentiation`
enables differentiable programming. This behavior is desirable.

Author: dan-zheng

test/AutoDiff/Sema/ExportedDifferentiationModule/Inputs/exports_differentiation.swift

@@ -0,0 +1 @@
+@_exported import _Differentiation

test/AutoDiff/Sema/ExportedDifferentiationModule/main.swift

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+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module %S/Inputs/exports_differentiation.swift -o %t/exports_differentiation.swiftmodule
+// RUN: %target-build-swift %s -I %t
+
+// Test whether importing a module with `@_exported import _Differentiation`
+// enables differentiable programming. This behavior is desirable.
+
+import exports_differentiation
+
+@differentiable
+func id<T: Differentiable>(_ x: T) -> T { x }