The attached test case from
https://github.com/llvm/llvm-project/issues/117294 used to cause an
assertion because we called classifPrim() on an array type.
The new result doesn't crash but isn't exactly perfect either. Since the
problem arises when evaluating an ImplicitValueInitExpr, we have no
proper source location to point to. Point to the caller instead.
If we have a placement-new destination already, use that instead of
allocating a new one.
Tests are partially based on
`test/SemaCXX/cxx2c-constexpr-placement-new.cpp`.
