PROGV doesn't rebind unbound vars with DEBUG 3

[Ivan Shvedunov]

Without (sb-ext:restrict-compiler-policy 'debug 3), the code
below works as expected and doesn't signal an error.

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* (sb-ext:restrict-compiler-policy 'debug 3)

((DEBUG . 3))
* (defvar *v*)

*V*
* (let ((*v* t)) (print *v*) (progv '(*v*) '() (print (boundp '*v*))) (print *v*))

T
NIL
debugger invoked on a UNBOUND-VARIABLE in thread
#<THREAD "main thread" RUNNING {1003E06533}>:
  The variable *V* is unbound.

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT] Exit debugger, returning to top level.

((LAMBDA ()))
   source: '*V*
0]

[Ivan Shvedunov (ivan4th)]

(sb-ext:restrict-compiler-policy 'debug 2) doesn't exhibit this problem.

[Stas Boukarev (stassats)]

cd7d708f21c4f7c800b0dbc6c038a3c1c31cec34