copy initialization of FieldDecl sometimes erroneously includes `= ` in source range

Source ranges for the initialization expression in a FieldDecl is inconsistent with that from VarDecl when copy initialization is used in some circumstances, e.g.

https://gcc.godbolt.org/z/vPaY3dM13

```c++
struct D {};
struct S {
D d0;
D d1 = d0; // CXXConstructExpr("= d0")
D d2 = {d0}; // CXXConstructExpr("= {d0}")
D d3 = D{d0}; // CXXTemporaryObjectExpr("D{d0}")
D d4{d0}; // CXXConstructExpr("{d0}")
};

void fn() {
D d0;
D d1 = d0; // CXXConstructExpr("d0")
D d2 = {d0}; // CXXConstructExpr("{d0}")
D d3 = D{d0}; // CXXTemporaryObjectExpr("D{d0}")
D d4{d0}; // CXXConstructExpr("{d0}")
}
```

It seems to be limited to CXXConstructExpr, but I've not looked exhaustively.