[clang-tidy] readability-simplify-boolean-expr should handle if statements with init statements

**Problem:** `readability-simplify-boolean-expr` doesn't simplify conditions in if statements with init statements.

**Examples:**
- `if (auto x = f(); x != nullptr)` → `if (auto x = f())`
- `if (bool b = g(); b == true)` → `if (bool b = g())`

**Request:** Extend the check to handle C++17 if-with-initializer syntax.