bug#44103: 28.0.50; [feature/native-comp] let-bound dynamic variables are optimized away

[Kisaragi Hiu]

--text follows this line--

Some functions use a dynamic (special, marked with defvar) variable to 
change their behaviors. In a lexically bound file, those dynamic 
variable bindings are currently optimized away.

# Reproduction

Run this function with the interpreter:

```elisp
(defun k/test ()
  (with-current-buffer (get-buffer-create "testing")
    (erase-buffer)
    (let ((data '((a . "b") (c . "b")))
          (json-encoding-pretty-print t))
      (insert (json-encode data)))))
```

the contents of #<buffer "testing"> after running `(k/test)` should be

```
{
  "a": "b",
  "c": "b"
}
```

whereas if you native-compile the code, the contents would become

```
{"a":"b","c":"b"}
```

because `json-encoding-pretty-print` is optimized away (I think).

# Potential fix

Maybe it's possible to check if a variable is dynamic (special / defined 
with defvar) and don't optimize them away?

# Related

This is also the core issue behind #43536.

In #43536, the reason the infinite recursion happened is because 
`org-roam-db-build-cache` prevents it by let-binding `org-mode-hook` to 
nil. Doing so, it ensures it's not called again recursively in 
`org-mode-hook` even if a user misconfigures Org-roam.



In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 
3.24.23, cairo version 1.17.3)
 of 2020-10-20 built on MF-PC
Repository revision: d5791ba5feeb5500433ca43506dda13c7c67ce14
Repository branch: makepkg
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Arch Linux

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