Fix llvm/lib/ObjCopy, llvm/llvm-ifs: c++20 compatibility

Cleanup: avoid referring to `std::vector<T>` members when `T` is incomplete.

This is [not legal](https://timsong-cpp.github.io/cppwp/n4868/vector#overview-4)
according to the C++ standard, and causes build errors in particular in C++20
mode. Fix it by defining the vector's type before using the vector.

Reviewed By: saugustine, MaskRay

Differential Revision: https://reviews.llvm.org/D135906