Issue Description:

It'd be useful for tools like SwiftLint to get information about closure expressions in source code.

Several rules currently need this information and obtain it using heuristics. It'd be way more reliable if this was available via SourceKit.

Some rules that could use it:

• Closure End Indentation

• Closure Parameter Position

• Closure Spacing

• Empty Parentheses with Trailing Closure

• Implicit Return

• Multiple Closures with Trailing Closure

• Trailing Closure

• Unneeded Parentheses in Closure Argument

• Unused Closure Parameter

• Vertical Parameter Alignment On Call

It'd be also useful to know whether a closure is a trailing closure inside a function call, but I'm not sure if it's semantically correct to add this information on this structure, because it's something available in several declarations such as CallExpr and SubscriptExpr, not ClosureExpr.