[AutoDiff upstream] Add Differentiable protocol derived conformances. (#30671)

Add `AdditiveArithmetic` derived conformances for structs and classes, gated by
the `-enable-experimental-differentiable-programming` flag.

Structs and classes whose stored properties all conform to `Differentiable` can
derive `Differentiable`:

- `associatedtype TangentVector: Differentiable & AdditiveArithmetic`
  - Member `TangentVector` structs are synthesized whose stored properties are
    all `var` stored properties that conform to `Differentiable` and that are
    not `@noDerivative`.
- `mutating func move(along: TangentVector)`

The `@noDerivative` attribute may be declared on stored properties to opt out of
inclusion in synthesized `TangentVector` structs.

Some stored properties cannot be used in `TangentVector` struct synthesis and
are implicitly marked as `@noDerivative`, with a warning:

- `let` stored properties.
  - These cannot be updated by `mutating func move(along: TangentVector)`.
- Non-`Differentiable`-conforming stored properties.

`@noDerivative` also implies `@_semantics("autodiff.nonvarying")`, which is
relevant for differentiable activity analysis.

Add type-checking and SILGen tests.

Resolves TF-845.

lib/Sema/CodeSynthesis.cpp

@@ -1369,3 +1369,21 @@ bool swift::hasLetStoredPropertyWithInitialValue(NominalTypeDecl *nominal) {
     return v->isLet() && v->hasInitialValue();
   });
 }
+
+void swift::addFixedLayoutAttr(NominalTypeDecl *nominal) {
+  auto &C = nominal->getASTContext();
+  // If nominal already has `@_fixed_layout`, return.
+  if (nominal->getAttrs().hasAttribute<FixedLayoutAttr>())
+    return;
+  auto access = nominal->getEffectiveAccess();
+  // If nominal does not have at least internal access, return.
+  if (access < AccessLevel::Internal)
+    return;
+  // If nominal is internal, it should have the `@usableFromInline` attribute.
+  if (access == AccessLevel::Internal &&
+      !nominal->getAttrs().hasAttribute<UsableFromInlineAttr>()) {
+    nominal->getAttrs().add(new (C) UsableFromInlineAttr(/*Implicit*/ true));
+  }
+  // Add `@_fixed_layout` to the nominal.
+  nominal->getAttrs().add(new (C) FixedLayoutAttr(/*Implicit*/ true));
+}