[Sema] Fix DerivedConformanceDifferentiable.cpp header order.

[dan-zheng]

Fixes breakages introduced in #29908.
Breakages due to not running tests after clang-format.
Breakages not caught because @swift-ci Please test tensorflow was down.

---

Fixed errors:

[2/4] Building CXX object lib/Sema/CMakeFil....dir/DerivedConformanceDifferentiable.cpp.o
FAILED: lib/Sema/CMakeFiles/swiftSema.dir/DerivedConformanceDifferentiable.cpp.o
...
In file included from /home/danielzheng/swift-dev/swift/lib/Sema/DerivedConformanceDifferentiable.cpp:21:
/home/danielzheng/swift-dev/swift/lib/Sema/DerivedConformances.h:372:10: error: unknown type name 'GuardStmt'
  static GuardStmt *returnIfNotEqualGuard(ASTContext &C, Expr *lhsExpr,
         ^
/home/danielzheng/swift-dev/swift/lib/Sema/DerivedConformances.h:376:10: error: unknown type name 'GuardStmt'
  static GuardStmt *returnFalseIfNotEqualGuard(ASTContext &C, Expr *lhsExpr,
         ^
/home/danielzheng/swift-dev/swift/lib/Sema/DerivedConformances.h:379:10: error: unknown type name 'GuardStmt'
  static GuardStmt *
         ^
/home/danielzheng/swift-dev/swift/lib/Sema/DerivedConformances.h:408:38: error: use of undeclared identifier 'ASTNode'
  convertEnumToIndex(SmallVectorImpl<ASTNode> &stmts, DeclContext *parentDC,
                                     ^
/home/danielzheng/swift-dev/swift/lib/Sema/DerivedConformances.h:413:32: error: unknown type name 'EnumElementDecl'
  enumElementPayloadSubpattern(EnumElementDecl *enumElementDecl, char varPrefix,
                               ^
5 errors generated.

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

@swift-ci Please test tensorflow still appears to be down.
ninja check-swift passes locally so I'll merge this now.