Allow std::initializer_list constructors in constexpr expressions

What does this implement/fix?

Previously attempting to declare a constexpr Eigen::Matrix/Eigen::Array would result in a compiler error, now it will succeed for fixed-size matrices as well as dynamic-sized ones with fixed storage size if all elements are initialized. This works when targeting C++20 and some basic functionality is also supported when using Clang targeting C++14/17, but GCC rejects declaring a matrix as constexpr until C++20.