[DWARFLinker] Handle empty sequences when processing DW_AT_LLVM_stmt_sequence attributes (#132875)

We previously assumed that every `DW_AT_LLVM_stmt_sequence` attribute
has a corresponding sequence in the processed line table. However, this
isn't always true. Some sequences can be removed by the linker if they
are empty, as shown
[here](https://github.com/alx32/llvm-project/blob/release/14.x/llvm/lib/DebugInfo/DWARF/DWARFDebugLine.cpp#L565-L566).
When an attribute refers to one of these removed sequences, there is no
actual sequence for it to match. In such cases, we update the attribute
to indicate that it is invalid and does not point to any sequence. This
informs readers that the attribute should be ignored.

The newly modified test would have triggered the assert that is being
removed in this patch.

Author: alx32

llvm/lib/DWARFLinker/Classic/DWARFLinker.cpp

@@ -2311,8 +2311,13 @@ void DWARFLinker::DIECloner::generateLineTableForUnit(CompileUnit &Unit) {
           uint64_t OrigStmtSeq = StmtSeq.get();
           // 1. Get the original row index from the stmt list offset.
           auto OrigRowIter = SeqOffToOrigRow.find(OrigStmtSeq);
-          assert(OrigRowIter != SeqOffToOrigRow.end() &&
-                 "Stmt list offset not found in sequence offsets map");
+          // Check whether we have an output sequence for the StmtSeq offset.
+          // Some sequences are discarded by the DWARFLinker if they are invalid
+          // (empty).
+          if (OrigRowIter == SeqOffToOrigRow.end()) {
+            StmtSeq.set(UINT64_MAX);
+            continue;
+          }
           size_t OrigRowIndex = OrigRowIter->second;
 
           // 2. Get the new row index from the original row index.

llvm/test/tools/dsymutil/ARM/stmt-seq-macho.test

@@ -41,12 +41,21 @@ ATTRIB int function2_copy2(int a) {
     int result = a - 22;
     return result;
 }
+
+struct logic_error {
+    logic_error(const char* s) {}
+};
+ 
+struct length_error : public logic_error {
+    __attribute__((noinline)) explicit length_error(const char* s) : logic_error(s) {}
+};
 
 int main() {
     int sum = 0;
     sum += function2_copy2(3);
     sum += function3_copy2(41);
     sum += function2_copy1(11);
+    length_error e("test");
     return sum;
 }
 EOF