[OpenMP] Fix incompatible attributes in OpenMPIRBuilder test.

Summary:
After D98740 the function attributes were changes to append rather than
overwrite when using the values defined in OMPKinds.def. One of the
OpenMPIRBuilder tests erroenously used `readnone` instead of `readonly` which
would cause a conflict and result in a broken module once `readonly` was
appended to it. This patch fixes the test to use the correct attributes.