[MC] Avoid calling vector<FieldInfo> members before FieldInfo is defined

The C++ Standard requires a complete type T when using any members of
`vector<T>`, see
https://eel.is/c++draft/vector#overview-4.

This only breaks with latest libc++ in C++20 mode and does not show up
in common configurations.
We have an internal experimental configuration that discovered this.

Reviewed By: alexfh

Differential Revision: https://reviews.llvm.org/D131595