[clang-tidy] Fix false positives involving type aliases in...

[clang-tidy] Fix false positives involving type aliases in `misc-unconventional-assign-operator` check

clang-tidy currently reports false positives even for simple cases such as:
```
struct S {
    using X = S;
    X &operator=(const X&) { return *this; }
};
```
This is due to the fact that the `misc-unconventional-assign-operator` check fails to look at the //canonical// types. This patch fixes this behavior.

Reviewed By: aaron.ballman, mizvekov

Differential Revision: https://reviews.llvm.org/D114197