Partially Revert #27862
When SE-110 was being implemented, we accidentally began to accept
closure parameter declarations that had no associated parameter names,
e.g.
foo { ([Int]) in /**/ }
This syntax has never been sanctioned by any version of Swift and should
be banned. However, the change was made long enough ago and there are
enough clients relying on this, that we cannot accept the source break
at the moment. For now, add a bit to ParamDecl that marks a parameter
as destructured, and back out setting the invalid bit on the type repr
for these kinds of declarations.
To prevent further spread of this syntax, stub in a warning that offers
to insert an anonymous parameter.
Resolves part of rdar://56673657 and improves QoI for errors like
rdar://56911630
