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
Since placeholder expansion works with a single placeholder, which is
somewhat at odds with multiple-trailing closures, we eagerly attempt to
expand all consecutive placeholders of closure type. That is, if the API
has multiple closure parameters at the end, expanding any one of them
will transform all of them to the new syntax.
Example
```
foo(a: <#T##()->()#>, b: <#T##()->()#>)
```
expanding *either* parameter will produce the following:
```
foo {
<#code#>
} b: {
<#code#>
}
```
(caveat: the indentation is not part of placeholder expansion, but it's
added here for clarity)
At least for now we do not attempt to corral an existing closure into
the new syntax, so for
```
foo(a: { bar() }, b: <#T##()->()#>)
```
The exansion will be
```
foo(a: { bar() }) {
<#code#>
}
```
as it was before.
rdar://59688632
