fix #10305 & #10699: FP unusedStructMember with aggregate initialization

[ludviggunne]

Compares number of non-static members with number of entries in braced initializer.

[olabetskyi]

10699 Can be added to title as it's pretty much the same.

[danmar]

In my humble opinion here is a better test case. There should be no "unusedStructMember" warning here:

struct S {
    int a;
};

S foo() {
    return S{0};
}

[ludviggunne]

I realize know that this doesn't actually solve the main in example in #10305 (only the simpler one in the comments), since it relies on the name of the struct appearing before the braced initalizer. So this will require some more work.