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Thank you for your contribution to GHC!

Please take a few moments to verify that your commits fulfill the following:

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• This MR implements the NoFieldSelectors proposal.

Summary of the changes

(Items with bold texts are the key parts of the implementation.)

• A new language extension, FieldSelectors, is added and enabled by default.
• NoFieldSelectors negates FieldSelectors.
• We added a new field in FieldLabel indicating whether FieldSelectors is enabled in the definition. As a result, we had to update the use sites of FieldLabel.
• Previously, whether DuplicateRecordFields is in effect is represented by Bool; we replaced it by an explicit datatype instead.
• lookupOccRn_overloaded has been renamed to lookupOccRn_overloaded_expr.
  • It is used to look up variables in expressions.
  • Selector functions created with NoFieldSelectors are hidden from this function.
• lookupGlobalOccRn_overloaded has been renamed to lookupGlobalOccRn_overloaded_pat.
  • It is used to find field names in record patterns, updates, and constructions.
  • It can find selector functions with NoFieldSelectors.
• Created a new datatype LookupOccRnOverloadedResult in place of Either Name [Name]
• Fixed the bug #18729 (closed) preventing it work on GHCi