Inline rename should be aware of unchangeable documents during conflict resolving

[genlu]

This was pointed out by @jasonmalinowski in this comment:
#40816 (review)

But if there's a conflict resolution in a file that otherwise didn't have a original location, I suspect we're going to try modifying the text buffer too.

Today, this is fine because opened documents can't be unchangeable, but this might change in the future depends on the design of code generator.