Don't enable diagnostics for CFI when setting the CFI sanitizer.

The SetSanitizer() method in cc/sanitize.go was also enabling
diagnostics for CFI when invoked from the sanitizer mutator. We
shouldn't be doing this, so this CL disables that.

Bug: 79785496
Test: strings libziparchive.so | grep 'ubsan'
Change-Id: Iffe0d4e9c333ac5dfbec3f48e381c232ce5d4463