Disallow (->) as a data constructor name (#16999)

The code was misusing isLexCon, which was never meant for validation.
In fact, its documentation states the following:

	Use these functions to figure what kind of name a 'FastString'
	represents; these functions do /not/ check that the identifier
	is valid.

Ha! This sign can't stop me because I can't read.

The fix is to use okConOcc instead. The other checks (isTcOcc or
isDataOcc) seem superfluous, so I also removed those.