[clang-tidy] Fix false positives with template in `misc-unconventional-assign-operator` check (#143292)
Fix false positives when copy assignment operator function in a template
class returns the result of another assignment to `*this`, this check
doesn't consider this situation that there will be a `BinaryOperator`
for assignment rather than `CXXOperatorCallExpr` since `this`'s type is
dependent.
Closes #143237.