As explained by commit 849f1dd15e92fda2b83dbb6144e6b28b2cb946e0,
-fxray-function-index was the original default but was accidentally flipped by
commit d8a8e5d6240a1db809cd95106910358e69bbf299. Restore the previous behavior.
Originally reported by Oleksii Lozovskyi in D145848.
Apply my post-commit comment on D81995. The negative name misguided commit
d8a8e5d6240a1db809cd95106910358e69bbf299 (`[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 .
