[DWARFLinker] Use different addresses to distinguish invalid DW_AT_LLVM_stmt_sequence offset (#149376)

It'd be helpful (especially when `llvm-dwarfdump ... | grep
<invalid_address>`) to separate two different invalid reasons for
debugging.

Author: DataCorrupted

llvm/lib/DWARFLinker/Classic/DWARFLinker.cpp

@@ -43,6 +43,12 @@ namespace llvm {
 using namespace dwarf_linker;
 using namespace dwarf_linker::classic;
 
+enum InvalidStmtSeqOffset {
+  MaxStmtSeqOffset = UINT64_MAX,
+  OrigOffsetMissing = MaxStmtSeqOffset - 1,
+  NewOffsetMissing = MaxStmtSeqOffset - 2,
+};
+
 /// Hold the input and output of the debug info size in bytes.
 struct DebugInfoSize {
   uint64_t Input;
@@ -2315,7 +2321,7 @@ void DWARFLinker::DIECloner::generateLineTableForUnit(CompileUnit &Unit) {
           // Some sequences are discarded by the DWARFLinker if they are invalid
           // (empty).
           if (OrigRowIter == SeqOffToOrigRow.end()) {
-            StmtSeq.set(UINT64_MAX);
+            StmtSeq.set(OrigOffsetMissing);
             continue;
           }
           size_t OrigRowIndex = OrigRowIter->second;
@@ -2325,7 +2331,7 @@ void DWARFLinker::DIECloner::generateLineTableForUnit(CompileUnit &Unit) {
           if (NewRowIter == OrigRowToNewRow.end()) {
             // If the original row index is not found in the map, update the
             // stmt_sequence attribute to the 'invalid offset' magic value.
-            StmtSeq.set(UINT64_MAX);
+            StmtSeq.set(NewOffsetMissing);
             continue;
           }