Fix issue 10198 CTFE struct Multidimensional block assignment

There are two bugs. (1) The code in todt.c ignored the possibility
that an initializer could be explicitly specified in the struct literal.
This is not the same as an initializer specified in the declaration of
the field.
This wrong behaviour was actually coded into a test in test42.d.
(2) The CTFE code didn't consider the multidimensional case.
Merge pull request #2094 from donc/ctfe10198multiblock

Fix issue 10198 CTFE struct Multidimensional block assignment