Changes the diagnostics emitted when an `@objc @implementation` extension is missing some of the members required by the extension:
• We now emit one error on the extension, plus a note for each missing member.
• Where possible, we also emit a note with a fix-it adding stubs.
For example:
```
9 | @objc @implementation extension ObjCClass {
| |- error: extension for main class interface does not provide all required implementations
| |- note: missing instance method 'method(fromHeader3:)'
| |- note: missing instance method 'method(fromHeader4:)'
| |- note: missing property 'propertyFromHeader7'
| |- note: missing property 'propertyFromHeader8'
| |- note: missing property 'propertyFromHeader9'
| |- note: missing instance method 'extensionMethod(fromHeader2:)'
| `- note: add stubs for missing '@implementation' requirements
```
With a fix-it on the last note to insert the following after the open brace:
```
@objc(methodFromHeader3:)
open func method(fromHeader3 param: Int32) {
<#code#>
}
@objc(methodFromHeader4:)
open func method(fromHeader4 param: Int32) {
<#code#>
}
@objc(propertyFromHeader7)
open var propertyFromHeader7: Int32 {
get {
<#code#>
}
set {
<#code#>
}
}
@objc(propertyFromHeader8)
open var propertyFromHeader8: Int32 {
get {
<#code#>
}
set {
<#code#>
}
}
@objc(propertyFromHeader9)
open var propertyFromHeader9: Int32 {
get {
<#code#>
}
set {
<#code#>
}
}
@objc(extensionMethodFromHeader2:)
open func extensionMethod(fromHeader2 param: Int32) {
<#code#>
}
```
Fixes rdar://130038221.
…for extensions. This change also removes @implementation(CategoryName); you should attach the category name to the @objc attribute instead. And there are small changes to how much checking the compiler will do on an @objc @implementation after the decl checker has discovered a problem with it.
• ObjCImplementation controls @implementation on extensions
• CImplementation controls @implementation and @_objcImplementation on cdecl functions
Why the difference between them? Because `@_objcImplementation extension` has already been adopted pretty widely, while `@_objcImplementation @_cdecl` is very new.
