subtype: Add a wrapper fast path to `jl_isa` (#64416)
Since #59221 a task can carry an inferred `CodeInstance`, and `start_task`
then goes through `jl_f_invoke`, which checks the closure argument against
the method signature. For a closure with type parameters that signature
element is the closure type's `name->wrapper`, which `jl_isa` could only
answer through full subtyping. This made single-threaded
`fetch(Threads.@spawn i)` roughly 30% slower than on 1.13.
Every instance of a datatype is an instance of its `name->wrapper`, so
`jl_isa` now returns true for that case directly. The check is positive
only; everything else falls through to the existing code. The same path
also speeds up other runtime `isa` checks against wrapper types such as
`x isa Array` or `x isa Base.RefValue`.
Assisted-by: Claude Code (Opus 5.5)
