[clang-tidy] Extend modernize-use-designated-initializers to cover per-member initialization

[chrchr-github]

struct S {
    int a;
    int b;
};

S f() {
    S s;
    s.a = 0;
    s.b = 1;
    //S s{ .a = 0, .b = 1 }; // fix-it
    return s;
}

https://godbolt.org/z/rGvrzasYn

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: None (chrchr-github)

~~~c++ struct S { int a; int b; };

S f() {
S s;
s.a = 0;
s.b = 1;
//S s{ .a = 0, .b = 1 }; // fix-it
return s;
}

https://godbolt.org/z/rGvrzasYn

</details>

[zeyi2]

I think it will be hard for clang-tidy to generate correct FIX-IT for some cases:

e.g.

struct S {
    int a;
    int b;
};

S f() {
    S s;
    int i = 1;
    s.a = 0;
    printf("%d", i++); //side effect
    s.b = i;
    return s;
}

struct S {
    int a;
    int b;
};

S f() {
    S s;
    int i = 1;
    s.a = 0;
    if (i) //control-flow
        s.b = 1;
    else
        s.b = 0;
    return s;
}

We may need to introduce some kind of flow-analysis for this. So I think maybe we can just match simple patterns where there are no intervening side effects or control-flow between the member assignments.

[vbvictor]

match simple patterns where there are no intervening side effects or control-flow between the member assignments.

We have bits of such logic in #158462.