TTG: Move SourceText from `Fixity` to `FixitySig`
It is only used there, simplifies the use of Fixity in the rest of
the code, and is moved into a TTG extension point.
Precedes !12842, to simplify it
It is only used there, simplifies the use of Fixity in the rest of
the code, and is moved into a TTG extension point.
Precedes !12842, to simplify it