Make buildable node fields mutable and fix withTrailingComma

[fwcd]

withTrailingComma currently discards the value for leadingTrivia. While adding the parameter would be an option, this PR refactors the function to use the

var result = self
result.field = value
return result

pattern. This also has the advantage that new optional fields cannot accidentally be forgotten.

To achieve this, the fields have to be mutable. Since we are dealing with value types with no further invariants, this should hopefully not be an issue though.

[fwcd]

@swift-ci please test

[ahoppen]

Sounds good. Thanks for fixing this.