Prefer default member initializers also for non-constant initializers

[N-Dekker]

Removed the restriction "for constant initializers" from guideline C.48, Prefer default member initializers to member initializers in constructors for constant initializers

• The restriction appears unnecessary, as discussed by issue C.48: Prefer default member initializers, also for non-constant initializers #2332, showing the following example, having a non-constant (now()) as default member initializer, which also appears preferable:

    class MyContainer {
        time_point<system_clock> creationTime{ system_clock::now() };  // Preferable, right?
        vector<int> data{};
    public:
        MyContainer() = default;
        explicit MyContainer(size_t n) : data(n) {}
        MyContainer(initializer_list<int> values) : data{ values } {}
    };

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• Follow-up to pull request Use function call in Example of C.48: Prefer default member initializer #2330 commit b057f29 (which was merged by Sergey Zubkov, @cubbimew)