Fix Trac #2723: keep track of record field names in the renamer

The idea here is that with -XNamedFieldPuns and -XRecordWildCards we don't
want to report shadowing errors for
	let fld = <blah> in C { .. }
But to suppress such shadowing errors, the renamer needs to know that
'fld' *is* a record selector.  Hence the new NameSet in
TcRnFypes.RecFieldEnv