Note [prefix_minus in negLitPred and negHashLitPred]

We want to parse -1 as a single token, but x-1 as three tokens.
So in negLitPred (and negHashLitPred) we require that we have a prefix
occurrence of the minus sign. See Note [Whitespace-sensitive operator parsing]
for a detailed definition of a prefix occurrence.

The condition for a prefix occurrence of an operator is:

  not precededByClosingToken && followedByOpeningToken

but we don't check followedByOpeningToken when parsing a negative literal.
It holds simply because we immediately lex a literal after the minus.