Concurrency: Avoid string interpolation in argument to fatalError().

In Embedded Swift, the `String` type is unavailable. This means that the
overload of `fatalError()` that takes a `String` is also unavailable. Specify a
`StaticString` in the setter for `TaskLocal.projectedValue` to satisfy this
constraint.

Author: tshortli

stdlib/public/Concurrency/TaskLocal.swift

@@ -311,7 +311,7 @@ public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible
 
     @available(*, unavailable, message: "use '$myTaskLocal.withValue(_:do:)' instead")
     set {
-      fatalError("Illegal attempt to set a \(Self.self) value, use `withValue(...) { ... }` instead.")
+      fatalError("Illegal attempt to set a TaskLocal value, use `withValue(...) { ... }` instead.")
     }
   }