Specify constructor template arguments for ConstexprTest struct
This should silence the following warning:
'ConstexprTest' may not intend to support class template argument deduction [-Werror,-Wctad-maybe-unsupported]
44 | constexpr ConstexprTest obj1(Matrix2d({{1, 2}, {3, 4}}));
| ^
third_party/eigen3/test/constexpr.cpp:14:8: note: add a deduction guide to suppress this warning