Duplicate statement diagnostics in result builder

[hamishknight]

The following gives a duplicate diagnostic:

@resultBuilder
struct Builder {
  static func buildBlock<T>(_ components: T...) -> T {
    components.first!
  }
  static func buildEither<T>(first component: T) -> T {
    component
  }
  static func buildEither<T>(second component: T) -> T {
    component
  }
}

func takesClosure(_ fn: () -> Void) -> Int? { nil }

@Builder
func foo() -> Int {
  if let x = takesClosure {} { // 2x warning: trailing closure in this context is confusable with the body of the statement
    0
  } else {
    1
  }
}