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

[Jeremy Morse via llvm-commits]

jmorse wrote:

Pushed up a merge to master to resolve conflicts:
 * In DwarfDebug.cpp some extra basic-block-sections logic has landed. I've placed the logic for "put prologue_end here" in `beginInstruction` before the basic-block-sections logic, as I believe that's guarding logic that considers what the previous linetable entry is.
 * empty-inline.mir has received some updates in the meantime; the diff from 'main' is now that prologue_end shifts to the first instruction with the scope-line, instead of appearing after some loads and branching. (Which is the whole point of this patch)

I believe these are all obvious fixes, will leave the patch here for a few hours to use the CI befor emerging.

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