Add constexpr, test for C++14 constexpr.
Reference issue
What does this implement/fix?
This is the next split-off from !881. It adds the C++14 parts of the patch which allows constexpr initialization of Maps from static constexpr memory and some of the basic operations (including, to my surprise, my memory is a sieve) addition and subtraction.
I included a testcase which is the testcase in !881 with the C++20 parts reduced to empty placeholders.
Edit: this is done. I realize now that I forgot to follow up on @cantonios comment re destructors ("Whatever we do for NVCC, we probably also need to do for HIP as well."), so I'll have to leave this as a draft until I can review what that implies. Edit: I think #if defined(EIGEN_GPUCC) should be the right thing.
Compared to the version in !881 this re-instates (I think) all the inline keywords that I had removed when adding constexpr.