Tweak mem categorization of upvar mutability

- Correctly categorize env pointer deref for `FnMut` as declared
  rather than inherited.  This fixes an assert in borrowck.
  Closes #18238
- Categorize env pointer deref as mutable only if the closure is
  `FnMut` *and* the original variable is declared mutable.  This
  disallows capture-by-value `FnMut` closures from mutating captured
  variables that aren't declared mutable.  This is a difference
  from the equivalent desugared code which would permit it, but
  it is consistent with the behavior of procs.  Closes #18335
- Avoid computing info about the env pointer if there isn't one.

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