Use @_borrowed on a few declarations in the stdlib and overlays.

Most of the stdlib's properties don't need @_borrowed because they're
@inlinable, but I did find one place in an overlay where it's probably
sensible to make the operation use generalized accessors even if
they're resilient.

Author: rjmccall

stdlib/public/SDK/CoreAudio/CoreAudio.swift

@@ -151,16 +151,17 @@ extension UnsafeMutableAudioBufferListPointer
   }
 
   /// Access an indexed `AudioBuffer` (`mBuffers[i]`).
+  @_borrowed
   public subscript(index: Index) -> Element {
-    get {
+    _read {
       precondition(index >= 0 && index < self.count,
         "subscript index out of range")
-      return (_audioBuffersPointer + index).pointee
+      yield ((_audioBuffersPointer + index).pointee)
     }
-    nonmutating set(newValue) {
+    nonmutating _modify {
       precondition(index >= 0 && index < self.count,
         "subscript index out of range")
-      (_audioBuffersPointer + index).pointee = newValue
+      yield (&(_audioBuffersPointer + index).pointee)
     }
   }
 }

stdlib/public/core/Collection.swift

@@ -411,6 +411,7 @@ public protocol Collection: Sequence where SubSequence: Collection {
   ///   `endIndex` property.
   ///
   /// - Complexity: O(1)
+  @_borrowed
   subscript(position: Index) -> Element { get }
 
   /// Accesses a contiguous subrange of the collection's elements.

stdlib/public/core/MutableCollection.swift

@@ -85,6 +85,7 @@ where SubSequence: MutableCollection
   ///   `endIndex` property.
   ///
   /// - Complexity: O(1)
+  @_borrowed
   override subscript(position: Index) -> Element { get set }
 
   /// Accesses a contiguous subrange of the collection's elements.