Clone a ClosureExpr and change all decl contexts within the body

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I suppose the motivating example isn't a huge problem, but it's definitely something that you'll have to look out for when implementing this.

The place where this transformation is not valid is in @_fixed_layout types in resilient modules, where a property initializer need to only reference public or @usableFromInline things.

@_fixed_layout
public struct X {
  public var x: (Int) -> Int = { $0 }
  public var y: (Int) -> Int = X.y_initialValue()
  static func y_initialValue() -> (Int) -> Int {
    return { $0 }
  }
}

Here, x is fine because the closure is directly written. But y calls something that's not @usableFromInline, meaning:

test.swift:4:34: error: static method 'y_initialValue()' is internal and cannot be referenced from a property initializer in a '@_fixed_layout' type
  public var y: (Int) -> Int = X.y_initialValue()
                                 ^
test.swift:5:15: note: static method 'y_initialValue()' is not '@usableFromInline' or public
  static func y_initialValue() -> (Int) -> Int {

Also, if the static function is going to be @inlinable or @_transparent, it may need to show up in parseable interface files, which would mean it would need a name other than "varName.initialValue"