fixes#24947
When injectdestructors detects that a variable is a tuple unpacking temp
(i.e. it is an `skTemp`, is not a cursor, and has tuple type) it does
not generate a destructor for it and only generates sink/bit assignments
for its components. However the reason it does not generate a destructor
is that it expects it to be fully unpacked, this is true for unpackings
in for loops but not for tuple unpacking assignments which supports `_`
since #22537. Tuple unpacking definitions for `var`/`let`/`const` do not
generate `skTemp` and use the same symbol kind as the definition so they
did not have this problem.
To keep this compatible, the `_` parts of the tuple unpacking
assignments are now not ignored and unpacked into `let _ = ...`, which
generates its own destructor. Another option might be to use `skLet`
instead of `skTemp` but this might cause changes to behavior like
additional copies, I am not sure about this though.
