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.
A new helper class DXILOpBuilder is added to create DXIL op function calls.
TableGen backend for DXILOperation will create table for DXIL op function parameter types.
When create DXIL op function, these parameter types will used to create the function type.
Reviewed By: bogner
Differential Revision: https://reviews.llvm.org/D130291
