Force the mangled name of MainActor.run to not include the Sendable constraint.

This API already shipped without the Sendable constraint, and adding the
constraint (while it's fine with the compiler) changed the mangled name of
this API. Use `@_silgen_name` to restore the old existing mangled name so
we don't break the ABI.

Fixes rdar://83644760.

Author: DougGregor

stdlib/public/Concurrency/MainActor.swift

@@ -45,6 +45,7 @@ import Swift
 @available(SwiftStdlib 5.5, *)
 extension MainActor {
   /// Execute the given body closure on the main actor.
+  @_silgen_name("$sScM3run10resultType4bodyxxm_xyYbKScMYcXEtYaKlFZ")
   public static func run<T: Sendable>(
     resultType: T.Type = T.self,
     body: @MainActor @Sendable () throws -> T