[clang] [analyzer] Workaround for slowdown spikes (unintended scope increase) (PR #136720)

[Balázs Benics via cfe-commits]

https://github.com/balazs-benics-sonarsource commented:

Looks good. There were two points unaddressed:
 - Finding a name for the flag without the `legacy-` prefix
 - Find out if we can ever have multiple root nodes in an exploded graph. 

https://github.com/llvm/llvm-project/pull/136720