[stdlib] Optional, Result: Try adding @_disfavoredOverload to work around a shadowing issue

The new ~Copyable generalizations have changed the function signature enough that alternative definitions of `map`/`flatMap` in existing code that used to be considered to shadow the originals no longer do so. This leads to use sites becoming ambiguous — a source break.

While we consider approaches to resolve this on the compiler side, let’s try slapping a `@_disfavoredOverload` on these and see if that helps.

(cherry picked from commit c6bc196)

Author: lorentey

stdlib/public/core/Optional.swift

@@ -186,6 +186,9 @@ extension Optional {
   /// - Returns: The result of the given closure. If this instance is `nil`,
   ///   returns `nil`.
   @_alwaysEmitIntoClient
+  @_disfavoredOverload // FIXME: Workaround for source compat issue with
+                       // functions that used to shadow the original map
+                       // (rdar://125016028)
   public func map<E: Error, U: ~Copyable>(
     _ transform: (Wrapped) throws(E) -> U
   ) throws(E) -> U? {
@@ -264,6 +267,9 @@ extension Optional {
   /// - Returns: The result of the given closure. If this instance is `nil`,
   ///   returns `nil`.
   @_alwaysEmitIntoClient
+  @_disfavoredOverload // FIXME: Workaround for source compat issue with
+                       // functions that used to shadow the original flatMap
+                       // (rdar://125016028)
   public func flatMap<E: Error, U: ~Copyable>(
     _ transform: (Wrapped) throws(E) -> U?
   ) throws(E) -> U? {

stdlib/public/core/Result.swift

@@ -49,6 +49,9 @@ extension Result {
   /// - Returns: A `Result` instance with the result of evaluating `transform`
   ///   as the new success value if this instance represents a success.
   @_alwaysEmitIntoClient
+  @_disfavoredOverload // FIXME: Workaround for source compat issue with
+                       // functions that used to shadow the original map
+                       // (rdar://125016028)
   public func map<NewSuccess: ~Copyable>(
     _ transform: (Success) -> NewSuccess
   ) -> Result<NewSuccess, Failure> {
@@ -187,6 +190,9 @@ extension Result {
   /// - Returns: A `Result` instance, either from the closure or the previous
   ///   `.failure`.
   @_alwaysEmitIntoClient
+  @_disfavoredOverload // FIXME: Workaround for source compat issue with
+                       // functions that used to shadow the original flatMap
+                       // (rdar://125016028)
   public func flatMap<NewSuccess: ~Copyable>(
     _ transform: (Success) -> Result<NewSuccess, Failure>
   ) -> Result<NewSuccess, Failure> {