DebugInfo: fix FSPL SILLocation assertion issue

We were reusing the SILLocation from return instructions
to generate projections to ultimately destroy values.

This fix improves on what we were doing before, by
converting the insertion point's SILLocation into a
RegularLocation _without dropping_ the source location.

If the SILLocation was tied to an ASTNode, it'll carry
over the line location for this new regular location.
Otherwise, it'll fallback to the prior strategy of
producing a line 0 autogenerated location.

resolves rdar://163281183

Author: kavon

lib/SIL/Utils/FieldSensitivePrunedLiveness.cpp

@@ -441,10 +441,23 @@ void TypeTreeLeafTypeRange::constructFilteredProjections(
         callback) {
   auto *fn = insertPt->getFunction();
   SILType type = value->getType();
-  auto loc =
-      (insertPt->getLoc().getKind() != SILLocation::ArtificialUnreachableKind)
-          ? insertPt->getLoc()
-          : RegularLocation::getAutoGeneratedLocation();
+
+  // Create a debug location appropriate for synthesizing projection
+  // instructions that satisfy `SILInstruction::verifyDebugInfo`, while
+  // trying to preserve debug info when it's valid to do so.
+  auto projectionLocFrom = [](SILInstruction *orig) -> SILLocation {
+    auto loc = orig->getLoc();
+    switch (loc.getKind()) {
+    case SILLocation::ReturnKind:
+    case SILLocation::ImplicitReturnKind:
+    case SILLocation::ArtificialUnreachableKind:
+      return RegularLocation::getDebugOnlyLocation(loc, orig->getModule());
+
+    default:
+      return loc;
+    }
+  };
+  SILLocation loc = projectionLocFrom(insertPt);
 
   PRUNED_LIVENESS_LOG(llvm::dbgs() << "ConstructFilteredProjection. Bv: "
                           << filterBitVector << '\n');