Change representation of field selector occurrences

This issue is a sub-task of issue #19461 ("Remove AmbiguousFieldOcc").

The task is to:

Change the names of the fields in type FieldOcc
Replace the use of AmbiguousFieldOcc p in HsRecFld to use FieldOcc p instead.

The change is spelled out in more detail in this note (section "Representation of field selector occurrences") in issue #19463 ("Refactor RecordUpd").

Edited May 02, 2021 by Shayne Fletcher

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