[llvm] [DWARF] Emit a worst-case prologue_end flag for pathological inputs (PR #107849)

[Jeremy Morse via llvm-commits]

jmorse wrote:

Rebased patch passes the online CI, and a ubsan stage2 build of clang completed without incident (assuming I've built it correctly), and didn't fire on the given reproducer. Possibly it's a case of mistaken commit identity, or composition with something else that landed at the time?

I'll re-land as the reported issues seem clean to me -- @hokein please do let me know if this reoccurs.

https://github.com/llvm/llvm-project/pull/107849