[NFC] Make dxil namespace consistent

We have namespaces `DXIL` and `dxil`, which is just confusing. This
renames `DXIL` -> `dxil` making everything consistent.

While the LLVM coding standards don't have a clear direction here, I
chose lower case because by my current unscientific count there are
more places where we had the lowercase namespace than the uppercase.