Desugar: allow reflection over internal APIs.

Desugar reflects over internal APIs at runtime, using the
java.lang.invoke.MethodHandles.Lookup API.

On OpenJDK 9 toolchains, such reflection is only allowed
to packages to which the java.lang.invoke module is opened.

This CL adds an override to open the module to all unnamed
modules (i.e., to Desugar) when running Desugar.

Test: make checkbuild
      (with OpenJDK 8u45 toolchain on the PATH)
Test: make EXPERIMENTAL_USE_OPENJDK9=true checkbuild
      (with OpenJDK 9-ea toolchain on the PATH)

Bug: 38177295

Change-Id: I2cf74a96ea17366dd50b8d92af8e41e812247ef7