[XRay] Rename XRayOmitFunctionIndex to XRayFunctionIndex

Apply my post-commit comment on D81995. The negative name misguided commit
d8a8e5d6 (`[clang][cli] Remove marshalling from
Opt{In,Out}FFlag`) to:

* accidentally flip the option to not emit the xray_fn_idx section.
* change -fno-xray-function-index (instead of -fxray-function-index) to emit xray_fn_idx

This patch renames XRayOmitFunctionIndex and makes -fxray-function-index emit
xray_fn_idx, but the default remains -fno-xray-function-index .