Incorrect member resolution for classes with multiple non-empty bases

[werat]

struct Base1 {
  int x;
  int y;
};

struct Base2 : Base1 {
};

struct Base3 {
  int z;
};

struct Foo : Base2, Base3 {
};

Foo foo;

Evaluating foo.z produces wrong result, likely because there's a bug somewhere in ParserContext::GetMemberInfo().