Improve ABI-only rethrowing map shims based on code review

Replace the hackish use of `@_disfavoredOverload` with the more principled
use of `@_silgen_name` for the entrypoint we are maintaining, then rename
these functions in source to `__rethrows_map` to indicate what they're for.

While here, make them `throws` instead of `rethrows`. The ABI is the
same, and `throws` allows us do avoid to do/catch tricks with rethrows
functions.

Author: DougGregor

stdlib/public/core/Collection.swift

@@ -1214,18 +1214,15 @@ extension Collection {
     return Array(result)
   }
 
-  // ABI-only entrypoint
+  // ABI-only entrypoint for the rethrows version of map, which has been
+  // superseded by the typed-throws version. Expressed as "throws", which is
+  // ABI-compatible with "rethrows".
   @usableFromInline
-  @_disfavoredOverload
-  func map<T>(
+  @_silgen_name("$sSlsE3mapySayqd__Gqd__7ElementQzKXEKlF")
+  func __rethrows_map<T>(
     _ transform: (Element) throws -> T
-  ) rethrows -> [T] {
-    do {
-      return try map(transform)
-    } catch {
-      try _rethrowsViaClosure { throw error }
-      Builtin.unreachable()
-    }
+  ) throws -> [T] {
+    try map(transform)
   }
 
   /// Returns a subsequence containing all but the given number of initial

stdlib/public/core/ExistentialCollection.swift

@@ -1334,18 +1334,15 @@ extension AnySequence {
     }
   }
 
-  // ABI-only entrypoint
+  // ABI-only entrypoint for the rethrows version of map, which has been
+  // superseded by the typed-throws version. Expressed as "throws", which is
+  // ABI-compatible with "rethrows".
   @usableFromInline
-  @_disfavoredOverload
-  func map<T>(
+  @_silgen_name("$ss11AnySequenceV3mapySayqd__Gqd__xKXEKlF")
+  func __rethrows_map<T>(
     _ transform: (Element) throws -> T
-  ) rethrows -> [T] {
-    do {
-      return try map(transform)
-    } catch {
-      try _rethrowsViaClosure { throw error }
-      Builtin.unreachable()
-    }
+  ) throws -> [T] {
+    try map(transform)
   }
 
   @inlinable
@@ -1440,18 +1437,15 @@ extension AnyCollection {
     }
   }
 
-  // ABI-only entrypoint
+  // ABI-only entrypoint for the rethrows version of map, which has been
+  // superseded by the typed-throws version. Expressed as "throws", which is
+  // ABI-compatible with "rethrows".
   @usableFromInline
-  @_disfavoredOverload
-  func map<T>(
+  @_silgen_name("$ss13AnyCollectionV3mapySayqd__Gqd__xKXEKlF")
+  func __rethrows_map<T>(
     _ transform: (Element) throws -> T
-  ) rethrows -> [T] {
-    do {
-      return try map(transform)
-    } catch {
-      try _rethrowsViaClosure { throw error }
-      Builtin.unreachable()
-    }
+  ) throws -> [T] {
+    try map(transform)
   }
 
   @inlinable
@@ -1552,18 +1546,15 @@ extension AnyBidirectionalCollection {
     }
   }
 
-  // ABI-only entrypoint
+  // ABI-only entrypoint for the rethrows version of map, which has been
+  // superseded by the typed-throws version. Expressed as "throws", which is
+  // ABI-compatible with "rethrows".
   @usableFromInline
-  @_disfavoredOverload
-  func map<T>(
+  @_silgen_name("$ss26AnyBidirectionalCollectionV3mapySayqd__Gqd__xKXEKlF")
+  func __rethrows_map<T>(
     _ transform: (Element) throws -> T
-  ) rethrows -> [T] {
-    do {
-      return try map(transform)
-    } catch {
-      try _rethrowsViaClosure { throw error }
-      Builtin.unreachable()
-    }
+  ) throws -> [T] {
+    try map(transform)
   }
 
   @inlinable
@@ -1666,18 +1657,15 @@ extension AnyRandomAccessCollection {
     }
   }
 
-  // ABI-only entrypoint
+  // ABI-only entrypoint for the rethrows version of map, which has been
+  // superseded by the typed-throws version. Expressed as "throws", which is
+  // ABI-compatible with "rethrows".
   @usableFromInline
-  @_disfavoredOverload
-  func map<T>(
+  @_silgen_name("$ss25AnyRandomAccessCollectionV3mapySayqd__Gqd__xKXEKlF")
+  func __rethrows_map<T>(
     _ transform: (Element) throws -> T
-  ) rethrows -> [T] {
-    do {
-      return try map(transform)
-    } catch {
-      try _rethrowsViaClosure { throw error }
-      Builtin.unreachable()
-    }
+  ) throws -> [T] {
+    try map(transform)
   }
 
   @inlinable

stdlib/public/core/Sequence.swift

@@ -691,18 +691,15 @@ extension Sequence {
     return Array(result)
   }
 
-  // ABI-only entrypoint
+  // ABI-only entrypoint for the rethrows version of map, which has been
+  // superseded by the typed-throws version. Expressed as "throws", which is
+  // ABI-compatible with "rethrows".
   @usableFromInline
-  @_disfavoredOverload
-  func map<T>(
+  @_silgen_name("$sSTsE3mapySayqd__Gqd__7ElementQzKXEKlF")
+  func __rethrows_map<T>(
     _ transform: (Element) throws -> T
-  ) rethrows -> [T] {
-    do {
-      return try map(transform)
-    } catch {
-      try _rethrowsViaClosure { throw error }
-      Builtin.unreachable()
-    }
+  ) throws -> [T] {
+    try map(transform)
   }
 
   /// Returns an array containing, in order, the elements of the sequence