[llvm] [DebugInfo] Preserve line and column number when merging debug info. (PR #129960)

[David Blaikie via llvm-commits]

dwblaikie wrote:

This seems like a pretty substantial departure from LLVM's documented approach to instruction line information - a policy that was established predominantly as an agreement/unified needs between Google (for profile accuracy) and Sony (for user debuggability/not so "jumpy" stepping).

So this deserves a bit more discussion to better understand why we're considering departing from that established decision now.

For instance, this seems like it does the wrong thing for PGO, as far I've understood the goals, for an example like this:
```
void f1();
void f2();
void f3();
void f4(bool b) {
  if (b) {
    f1();
    f2();
  } else {
    f1();
    f3();
  }
}
```
The call to `f1()` is hoisted out of the if/else, and currently would be given a source location of line zero - whereas with this change, it receives line 6 (the first of the two calls).

The premise so far has been that that change is harmful for users (they might incorrectly conclude that since line 6 is reached, then `b` must be true, even when it is false) and for profile-guided optimizations (for similar reasons - the compiler might assume that `b` is true, that the `f2` call is similarly hot, and the `f3` call is cold).

Could you provide more context for how this ^ benefits PGO? Or which cases do benefit?

https://github.com/llvm/llvm-project/pull/129960