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

~~~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

Start by locating the clang-tidy modernize-use-designated-initializers check and its existing tests. Compare the shown per-member assignments with the check's current supported cases; done means it emits the requested designated-initializer fix-it for this pattern and tests cover it.

Written by the indexing model from the issue text.