clang-tidy v. -Weffc++

Nobody has claimed this yet.

## Context

Recent versions of clang-tidy added `readability-redundant-member-init`: https://clang.llvm.org/extra/clang-tidy/checks/readability-redundant-member-init.html

This is handy! When a class member has a default initializer it is redundent to initialize it in the member list.

But, because some types don't have default initializers, forgetting to initialize them in the member list (or using [c++11 initialization in the class definition](https://arne-mertz.de/2015/08/new-c-features-default-initializers-for-member-variables/)) can lead to serious trouble like mapbox/wagyu#69 - refs mapbox/wagyu#70.

So, this is the reason, at https://github.com/mapbox/cpp/issues/37#issuecomment-336200744, that we recommend using g++ and the `-Weffc++` flag because it can catch this (note, clang++ plus `-Weffc++` cannot):

> -Weffc++ - useful when building with g++ (does not do much with clang++). With g++ it can catch uninitialized class members and prevent crashes like mapbox/wagyu#69 - refs mapbox/wagyu#70

## Problem

- `-Weffc++` will warn on all class members not explicitly initialized in the initializer list
- `clang-tidy` will automatically remove variables from the initializer list that have default constructors

So, the two will fight: causing each other warnings. For this reason I think we should likely:

- let clang-tidy win
- recommend no longer using g++ with `-Weffc++`
- figure out what alternative way we can catch when members, without default initializers, are uninitialized (without needing to use `-Weffc++`). Maybe another `clang-tidy` check?

Start with clang-tidy's readability-redundant-member-init documentation and the linked mapbox/cpp#37, wagyu#69, and wagyu#70 discussions; no repository file or test is identified. Determine an agreed approach for -Weffc++, clang-tidy guidance, and detecting uninitialized members; done requires a documented recommendation and configuration.

Written by the indexing model from the issue text.