[Parse] Recover slightly better from bad shorthand `if let`
Instead of assuming that `if let <expr>` is meant
to be `if case <expr> = ...`, turn it into
`if let _ = <expr>`, which is consistent with
the fix-it we suggest.
This currently doesn't have much of an effect on
the diagnostics we produce, but will be important
once we start doing bidirectional inference for
ExprPatterns, as it avoids unhelpful diagnostics.
