Update some RenderNode constants to be mutable

[mayaepps]

Bug/issue #, if applicable: rdar://164748009

Summary

This updates some of the properties in the RenderNode tree to be mutable. A number of the top-level properties in RenderNode are already mutable, and making these additional properties mutable as well allows callers to modify these values.

To determine which properties it makes sense to make mutable, I followed the general pattern that if the property was created by information extracted from the source symbol (via symbol graphs), such as type information about a REST symbol, it shouldn’t be allowed to mutate. This information is unlikely to change without changing the shape of the symbol being documented. Similarly, properties that represent a unique identifier for the overall structure should also not be mutable.

Dependencies

N/A

Testing

Create a RenderNode object, and confirm that you can update one of the previously-immutable types in it, such as RenderBlockContent.Term.inlineContent without compilation errors.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [n/a] Added tests
• Ran the ./bin/test script and it succeeded
• [n/a] Updated documentation if necessary