Fix a debug location in DCE.

Fixes SR-15300: Compiler crash when using Builtin.unreachable in
initializers

Otherwise, this triggers an debug info verification assert that
invalid locations must only be on unreachable instructions.

This is why generating lifetime cleanup code should never inherit its
location from its insertion point.

Author: atrick

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -581,9 +581,11 @@ bool DCE::removeDead() {
         // dead and will be deleted.
         // We do not have to end lifetime of a @guaranteed non phi arg.
         if (arg->getOwnershipKind() == OwnershipKind::Owned) {
+          auto loc = RegularLocation::getAutoGeneratedLocation();
+          // insertPt is non-null because Undef is non-owned.
           auto insertPt = getInsertAfterPoint(arg).getValue();
           SILBuilderWithScope builder(insertPt);
-          auto *destroy = builder.createDestroyValue(insertPt->getLoc(), arg);
+          auto *destroy = builder.createDestroyValue(loc, arg);
           LiveInstructions.insert(destroy);
         }
         i++;