[DebugInfo] When merging locations prefer unannotated empty locs

When merging DILocations, we prefer to use DebugLoc::getMergedLocation when
possible to better preserve DebugLoc coverage tracking information through
transformations (as conversion to DILocations drops all coverage tracking
data). Currently, DebugLoc::getMergedLocation checks to see if either
DebugLoc is empty and returns it directly if so, to propagate that
DebugLoc's coverage tracking data to the merged location; however, it only
checks whether either location is valid, not whether they are annotated.

This is significant because an annotated location is not a bug, while an
empty unannotated location may be one; therefore, we check to see if either
location is unannotated, and prefer to return that location if it exists
rather than an annotated one.

This change is NFC outside of DebugLoc coverage tracking builds.

Author: SLTozer

llvm/lib/IR/DebugLoc.cpp

@@ -181,10 +181,19 @@ DebugLoc DebugLoc::getMergedLocations(ArrayRef<DebugLoc> Locs) {
   return Merged;
 }
 DebugLoc DebugLoc::getMergedLocation(DebugLoc LocA, DebugLoc LocB) {
-  if (!LocA)
-    return LocA;
-  if (!LocB)
+  if (!LocA || !LocB) {
+    // If coverage tracking is enabled, prioritize returning empty non-annotated
+    // locations to empty annotated locations.
+#if LLVM_ENABLE_DEBUGLOC_TRACKING_COVERAGE
+    if (!LocA && LocA.getKind() == DebugLocKind::Normal)
+      return LocA;
+    if (!LocB && LocB.getKind() == DebugLocKind::Normal)
+      return LocB;
+#endif // LLVM_ENABLE_DEBUGLOC_TRACKING_COVERAGE
+    if (!LocA)
+      return LocA;
     return LocB;
+  }
   return DILocation::getMergedLocation(LocA, LocB);
 }