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
[[noreturn]] can be used since Oct 2016 when the minimum compiler requirement was bumped to GCC 4.8/MSVC 2015.
Note: the definition of LLVM_ATTRIBUTE_NORETURN is kept for now.
Angelo Matni