[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