[gardening] Ensure functions use stable hashes across reload

Root cause is that `Function::Hash()` depends on the owner class
id. This causes two issues

a) Issue with reload:

During reload the reloader code will change `Function::owner()`
from a `Class` to a `PatchClass`. This in return will cause a change in
the function's hash, which means we won't find the function in the
closure cache anymore.

b) Issue with app-jit:

As part of app-jit we re-number class ids. This re-numbering would also
render the closure cache as invalid - as the function hash code would
change. So instead the closure cache was cleared entirely. While this
works for AOT, in app-jit we may serialize closure functions and their
code. Though at app-jit runtime we then fail to find the functions in
the closure function cache (as it was cleared during snapshoting)

(This will surface by us triggering an optimizing compile of a function
in the background compiler which will try to make a new `Function`
object for a closure, even though the app-jit snapshot already has one)

To fix these issues we make the `Function::Hash()` stable across reload

  * we use `Function::Owner()` instead of `Function::owner()` (the
    former will always return the actual class, the ladder can return
    the `PatchClass`)

  * we avoid re-hashing the closure cache by making `Function::Hash()`
    use the name of the class instead of the class id

  * we clear out the closure cache array during AOT snapshoting, as this
    is not used in AOT runtime.

Closes https://github.com/dart-lang/sdk/issues/52803

TEST=Fixes various issues on CI.

Change-Id: I352d0a768df0f29d504cdd80e3533cbedc437b9b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/311744
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>