Fix parsing error for function with implicit return of closure expression

Reproducer which fail to parse:

func fun() -> () -> Int { { return 42 } }

which is equivalent to:

func fun() -> () -> Int { return { return 42 } }

From https://docs.swift.org/swift-book/LanguageGuide/Functions.html:

Functions With an Implicit Return
If the entire body of the function is a single expression, the function implicitly returns that expression.

This parsing error was caused by disambiguation introduced by SONARSWIFT-11, it should be safe to allow code blocks with single element being a closure expression.