[AccelTable][nfc] Add helper function to cast AccelTableData (#77100)

Specializations of AccelTableBase are always interested in accessing the
derived versions of their data classes (e.g. DWARF5AccelTableData). They
do so by sprinkling `static_casts` all over the code.

This commit adds a helper function to simplify this process, reducinng
the number of casts that have to be made in the middle of code, making
it easier to read.