Remove API notes compilation support from swiftc (#21759)

We don't use this feature anymore (see 0149129ed), and now that we've
branched Swift 5.0 for the last time it's time to rip it out of swiftc
(and then from Clang).

No functionality change, unless someone was compiling their own API
notes. If you were doing that, just copy them into place as text.

test/APINotes/Inputs/roundtrip.apinotes

@@ -1,79 +0,0 @@
----
-Name:            AppKit
-Availability:    available
-AvailabilityMsg: ''
-Classes:
-  - Name:            NSCell
-    Availability:    available
-    AvailabilityMsg: ''
-    Methods:
-      - Selector:        init
-        MethodKind:      Instance
-        NullabilityOfRet: U
-        Availability:    available
-        AvailabilityMsg: ''
-        DesignatedInit:  true
-      - Selector:        'initImageCell:'
-        MethodKind:      Instance
-        Nullability:     [ N ]
-        NullabilityOfRet: U
-        Availability:    available
-        AvailabilityMsg: ''
-        DesignatedInit:  true
-      - Selector:        'initTextCell:'
-        MethodKind:      Instance
-        Nullability:     [ N ]
-        NullabilityOfRet: U
-        Availability:    available
-        AvailabilityMsg: ''
-        DesignatedInit:  true
-      - Selector:        'initWithCoder:'
-        MethodKind:      Instance
-        Nullability:     [ N ]
-        NullabilityOfRet: U
-        Availability:    available
-        AvailabilityMsg: ''
-        DesignatedInit:  true
-        Required:        true
-  - Name:            NSView
-    AuditedForNullability: true
-    Availability:    available
-    AvailabilityMsg: ''
-    Methods:
-      - Selector:        'addSubview:'
-        MethodKind:      Instance
-        Nullability:     [ N ]
-        NullabilityOfRet: N
-        Availability:    available
-        AvailabilityMsg: ''
-      - Selector:        'addSubview:positioned:relativeTo:'
-        MethodKind:      Instance
-        Nullability:     [ N, N, O ]
-        NullabilityOfRet: N
-        Availability:    available
-        AvailabilityMsg: ''
-      - Selector:        'beginDraggingSessionWithItems:event:source:'
-        MethodKind:      Instance
-        Nullability:     [ U, U, N ]
-        NullabilityOfRet: N
-        Availability:    available
-        AvailabilityMsg: ''
-    Properties:
-      - Name:            enclosingScrollView
-        Nullability:     O
-        Availability:    available
-        AvailabilityMsg: ''
-      - Name:            makeBackingLayer
-        Nullability:     N
-        Availability:    available
-        AvailabilityMsg: ''
-Functions:
-  - Name:            NSAvailableWindowDepths
-    NullabilityOfRet: N
-    Availability:    available
-    AvailabilityMsg: ''
-Globals:
-  - Name:            NSCalibratedWhiteColorSpace
-    Nullability:     N
-    Availability:    available
-    AvailabilityMsg: ''

test/APINotes/yaml-roundtrip.swift

@@ -1,7 +0,0 @@
-# RUN: %swift_driver_plain -apinotes -yaml-to-binary -o %t.apinotesc %S/Inputs/roundtrip.apinotes
-# RUN: %swift_driver_plain -apinotes -binary-to-yaml -o %t.apinotes %t.apinotesc
-
-# Note: We don't diff the results because they will change as Clang's
-# API notes support evolves, and we don't want to tie that closely to
-# a specific Clang.
-