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

[Reid Kleckner via llvm-commits]

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@@ -125,6 +132,14 @@ DILocation *DILocation::getMergedLocation(DILocation *LocA, DILocation *LocB) {
   if (LocA == LocB)
     return LocA;
 
+  if (PreserveMergedDebugInfo) {
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rnk wrote:

Please add a "why" comment here. Suggested text:
  // For some use cases (SamplePGO), it is important to retain distinct source locations. When this flag is set, we choose arbitrarily between A and B, rather than computing a merged location using line 0, which is typically not useful for PGO.

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