don't let class be merged if it has field been marked as can't rename
don't let class be merged if it has field been marked as can't rename

Summary: Such fields might be used in reflection, so we should avoid the class being merged into another class and cause the field's name change. (Note kConditional means other class can still be merged into this class)

Reviewed By: wsanville

Differential Revision: D39479769

fbshipit-source-id: 5e30597ad8a04caaa4878b6e008490abd239942c

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