Fix syntaxError on lambda inside decltype

[gerboengels]

Got a syntax error on the following use case:

void f() {
  using customComparator = decltype([] (const X& lhs, const X& rhs) { return lhs.CompareTo(rhs); });
  std::map<X, int, customComparator> m;
}

The issue being that the semicolon inside the lambda was matched as the end of the using-statement, instead of the semicolon at the end of the line

This PR fixes this case