[clang][NFCI] Get rid of ConstantMatrixTypeBitfields to avoid increasing the size of every type.

sizeof(ConstantMatrixTypeBitfields) > 8 which increases the size of every type.
This was not detected because no corresponding static_assert for its size was
added.

To prevent this from occuring again replace the various static_asserts for
the size of each of the bit-field classes by a single static_assert for the
size of Type.

I have left ConstantMatrixType::MaxElementsPerDimension unchanged since
the limit is exercised by multiple tests.