[llvm] [DWARF] Emit a minimal line-table for totally empty functions (PR #107267)

[Jeremy Morse via llvm-commits]

jmorse wrote:

Rebased, then discovered via another test that the scope-line is going to be terminated by the second basic block in the program. See the new additions to the test I'm adding, we'll by default put in an explicit line-zero location, specifically to avoid the scope-line running through the whole program.

I'd missed that this would happen; I still feel it'd be worthwhile to emit this scope line on the entry block, at the very least so that people can set a breakpoint on the function. (Re-requesting review in case this shifts peoples opinions).

https://github.com/llvm/llvm-project/pull/107267