Unnecessary error when declaring extension to type with @available annotation

[NachoSoto]

Describe the bug
Declaring extensions to types probably should inherit the same availability as the type itself.

Steps To Reproduce
I expect the following code to compile with no issues:

@available(iOS 13.0, *)
class A {}

extension A {}

Expected behavior
The functions declared in the extension are only available on iOS 13.0 and forward without needing to duplicate the @available annotation.

Actual behavior

'A' is only available in application extensions for iOS 13.0 or newer
Add @available attribute to enclosing extension

Environment (please fill out the following information)

• Swift: 5.7 (swiftlang-5.7.0.127.4 clang-1400.0.29.50)
• OS: macOS 13.0
• Xcode: 14.0 RC