clang-tidy v. -Weffc++

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## 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?

Iniziare dalla documentazione di clang-tidy's readability-redundant-member-init e dalle discussioni collegate mapbox/cpp#37, wagyu#69 e wagyu#70; non sono identificati file o test del repository. Determinare un approccio condiviso per -Weffc++, le indicazioni di clang-tidy e il rilevamento dei membri non inizializzati; il lavoro è completo quando sono disponibili una raccomandazione e una configurazione documentate.

Scritto dal modello di indicizzazione a partire dal testo della issue.