[AutoDiff] Enable cross-file derivative registration.

Lift temporary cross-file derivative registration restriction.

`@derivative` attribute type-checking simplications coming soon: TF-1099.
Original function and derivative function must have same access level, with one
exception: public original functions may have internal `@usableFromInline`
derivatives.

Author: dan-zheng

lib/Sema/TypeCheckAttr.cpp

@@ -4423,14 +4423,6 @@ static bool typeCheckDerivativeAttr(ASTContext &Ctx, Decl *D,
     return true;
   }
 
-  // Reject different-file derivative registration.
-  // TODO(TF-1021): Lift same-file derivative registration restriction.
-  if (originalAFD->getParentSourceFile() != derivative->getParentSourceFile()) {
-    diags.diagnose(attr->getLocation(),
-                   diag::derivative_attr_not_in_same_file_as_original);
-    return true;
-  }
-
   // Reject duplicate `@derivative` attributes.
   auto &derivativeAttrs = Ctx.DerivativeAttrs[std::make_tuple(
       originalAFD, resolvedDiffParamIndices, kind)];

test/AutoDiff/Sema/derivative_attr_type_checking.swift

@@ -743,11 +743,9 @@ extension InoutParameters {
   }
 }
 
-// Test cross-file derivative registration. Currently unsupported.
-// TODO(TF-1021): Lift this restriction.
+// Test cross-file derivative registration.
 
 extension FloatingPoint where Self: Differentiable {
-  // expected-error @+1 {{derivative not in the same file as the original function}}
   @derivative(of: rounded)
   func vjpRounded() -> (
     value: Self,