Return type in the closure signature is often redundant when expanding
placeholders, because the type of the clossures are usually inferred
from the context (i.e. calling function), users don't need to write the
return type explicitly.
They are not only redundant, but also sometimes harmful when the return
type is a generic parameter or its requirement. Actually, there is no
correct spelling in such cases.
So omit the return type and the parentheses around the parameter clause.
rdar://problem/63607976
When completing a single argument for a trailing closure, pre-expand the
closure expression syntax instead of using a placeholder. It's not valid
to pass a non-closure anyway.
rdar://62189182
