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

[via cfe-commits]

vabridgers wrote:

> The tests only utilize assignments involving literals. It should also test something like this:
> 
> ```c++
> int func();
> 
> void test() {
>     int i = func();
>     if (true) {
>         i = 0;
>     }
> }
> ```
> 
> Since it is not known if 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).

I believe I've address this and added a test case.

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