[clang] Use enum for LangOptions::SYCLVersion instead of unsigned

`LangOptions::SYCLVersion` can only have two values. This patch introduces an enum that allows us to reduce the member size from 32 bits to 1 bit.

Consequently, this also makes marshalling of this option fit into our model for enums: D84674.

Reviewed By: bader

Differential Revision: https://reviews.llvm.org/D93540