bug#69108: false-positive warning "variable ‘_’ not left unused" in if-let* and if-let

[Konstantin Kharlamov]

I've been writing an answer for a question on emacs.stackexchange¹ and to avoid
nested `if` and `let` clauses I used a `if-let*`, and result of one of the 
checks I
assigned to a `_` variable, because the variable would be left unused, it's 
only the
check being non-nil that mattered.

But when byte-compiled that triggered a:

    test.el:6:9: Warning: variable ‘_’ not left unused

…which is untrue, because it is unused.

The problem is present in both `if-let` and `if-let*`

# Steps to reproduce

1. Create test.el with the following code:

    ;;; -*- lexical-binding: t -*-
    (if-let*
        ((_ nil))
        (print "then clause")
      (print "else clause"))

2. M-x byte-compile test.el

## Expected

It byte-compiles with no warnings

## Actual

It compiles with a warning:

    test.el:3:7: Warning: variable ‘_’ not left unused

# Additional information

Emacs version: commit d4d5830f8a0 built two weeks ago from master.

1: https://emacs.stackexchange.com/questions/80351/delete-prettify-symbol