[stdlib] improve accessor declarations

Author: glessard

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -587,9 +587,10 @@ extension Unsafe${Mutable}BufferPointer where Element: ~Copyable {
 
   @unsafe
   @available(SwiftStdlib 6.2, *)
+  @_alwaysEmitIntoClient
   public var mutableSpan: MutableSpan<Element> {
     @lifetime(borrow self)
-    @_alwaysEmitIntoClient
+    @_transparent
     get {
       unsafe MutableSpan(_unsafeElements: self)
     }

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -1164,9 +1164,10 @@ extension Unsafe${Mutable}RawBufferPointer {
 
   @unsafe
   @available(SwiftStdlib 6.2, *)
+  @_alwaysEmitIntoClient
   public var mutableBytes: MutableRawSpan {
     @lifetime(borrow self)
-    @_alwaysEmitIntoClient
+    @_transparent
     get {
       unsafe MutableRawSpan(_unsafeBytes: self)
     }