[SYCL] Account for -ffine-grained-bitfield-accesses (#23007)
When community added support for [[gcc_struct]], the code flow path in
accumulateBitFields changed. On Windows, we began to always try to merge
contiguous bitfields of the same type into one storage block. As a
result, the bit-field "split" that was otherwise performed when the
option -ffine-grained-bitfield-accesses was enabled, was no longer done.
This leads to performance differences for some cases.
