Fix handling of unboxed closure type param substitutions

- When selecting an implicit trait impl for an unboxed closure, plumb
  through and use the substitutions from impl selection instead of
  using those from the current param environment in trans, which may
  be incorrect.
- When generating a function declaration for an unboxed closure, plumb
  through the substitutions from the param environment of the closure
  as above.  Also normalize the type to avoid generating duplicate
  declarations due to regions being inconsistently replaced with
  ReStatic elsewhere.
- Do not place the closure type in the self param space when
  translating the unboxed closure callee, etc.  It is not actually
  used, and doing so conflicts with the self substitution from
  default trait methods.

Closes #18661
Closes #18685

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