[SYCL] Refactor accessor handling

[elizabethandrews]

There are a couple of issues with how accessors are handled -

1. There is a discrepancy in how inherited accessors are handled - SYCLSimdAccessorPtrAttr is not handled. Is this correct?
2. Accessor diagnostics do not set IsInvalid and therefore does not exit early in case of errors
3. Code is messy in general mixed in with handleSpecialType.

[elizabethandrews]

Point 2 in description was fixed in elizabethandrews@6871143

Point 1 and 3 were fixed during refactoring in #3892.