[clang-tools-extra] [clang-tidy] Add new check 'misc-scope-reduction' (PR #175429)

[Oliver Stöneberg via cfe-commits]

firewave wrote:

The tests only utilize assignments involving literals. It should also test something like this:

```cpp
int func();

void test() {
    int i = func();
    if (true) {
        i = 0;
    }
}
```

Since it is not known of the call has any side effects it should not be suggested. If the implementation were visible it could be determined but I think such assignments should be completely excluded for now and left for a follow-up (and have a tunable).

https://github.com/llvm/llvm-project/pull/175429