Allow record wildcards for empty records

Creating this as a tracking ticket for the implementation of https://github.com/ghc-proposals/ghc-proposals/pull/496, which is already accepted.

The idea is to allow

data Foo = Foo { }

f (Foo {..}) = 1

x = Foo {..}

for consistency with non-empty records. Other than aesthetic reasons, this is also practically useful for code generation - you can treat record matching with a wildcard the same way, regardless of how many fields the record being matched has.

Notably,

data Foo = Foo
f (Foo {..}) = 42

is also allowed, despite the inconsistency it causes with regards to having to now differentiate between n = 0 and n > 0, where n is the number of arguments the constructor Foo has.

This seems to have been briefly discussed in the linked proposal.

To summarise, the new behaviour should be:

Only reject a wildcard when the constructor it's attached to is not a record and has at least one argument/field.

I'll be implementing this.

Implementation notes: The error that's currently produced is GHC.Tc.Errors.Types.TcRnIllegalWildcardsInConstructor. The comment attached to it should be amended to reflect the new reality.

It's produced only in GHC.Rename.Pat.rnHsRecFields.rn_dotdot. The check there needs to be amended to also find out if the constructor has non-record arguments.