[MC] NFC. Avoid redundant copies when constructing StructFieldInfo

Follow-up after D131595, see comments in the review thread.

The intention of having two constructors was to minimize the copies of
`vector`, but a lack of `std::move` on the call site caused the wrong
constructor to be called.

Switched to a single constructor that accepts a value.
Accepting by value allows to have a single constructor and still decide
to copy or move on the call site.