[SYCL-MLIR][NFC] Use clang::Builtin enum when possible (#7257)

1. Used `clang::Builtin` enum instead of string comparison when possible
2. Moved some builtin handling to `EmitBuiltinOps()`
3. Refactored `EmitBuiltinOps()`

Signed-off-by: Tsang, Whitney <[email protected]>

Author: whitneywhtsang