Use reference only for DW_FORM_ref*

[jankratochvil]

Summary

D53530 could interpret DW_AT_count in DW_FORM_data1 as a DIE reference as it did not verify its form. Verify that reference really uses only DW_FORM_ref* forms as it could otherwise accidentally follow a constant number as DIE offset. That fixes a regression as described there.
That DW_FORM_ref_sig8 comment should be updated by D54670 after it gets approved.

[labath]

btw, I have a patch for the same bug here: D55991

[jankratochvil]

It is a duplicate of D55991.

lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugInfoEntry.cpp

 // relative offsets as needed.
 //----------------------------------------------------------------------
 uint64_t DWARFDebugInfoEntry::GetAttributeValueAsReference(
     SymbolFileDWARF *dwarf2Data, const DWARFUnit *cu,
     const dw_attr_t attr, uint64_t fail_value,
     bool check_specification_or_abstract_origin) const {
   DWARFFormValue form_value;
   if (GetAttributeValue(dwarf2Data, cu, attr, form_value, nullptr,
-                        check_specification_or_abstract_origin))
+                        check_specification_or_abstract_origin)
+      && form_value.FormIsReference())
     return form_value.Reference();
   return fail_value;
 }
 
 uint64_t DWARFDebugInfoEntry::GetAttributeValueAsAddress(
     SymbolFileDWARF *dwarf2Data, const DWARFUnit *cu,
     const dw_attr_t attr, uint64_t fail_value,
     bool check_specification_or_abstract_origin) const {

lldb/source/Plugins/SymbolFile/DWARF/DWARFFormValue.h

public:
   const ValueType &Value() const { return m_value; }
   ValueType &ValueRef() { return m_value; }
   void SetValue(const ValueType &val) { m_value = val; }
 
   void Dump(lldb_private::Stream &s) const;
   bool ExtractValue(const lldb_private::DWARFDataExtractor &data,
                     lldb::offset_t *offset_ptr);
   const uint8_t *BlockData() const;
+  bool FormIsReference() const;
   uint64_t Reference() const;
   uint64_t Reference(dw_offset_t offset) const;
   bool Boolean() const { return m_value.value.uval != 0; }
   uint64_t Unsigned() const { return m_value.value.uval; }
   void SetUnsigned(uint64_t uval) { m_value.value.uval = uval; }
   int64_t Signed() const { return m_value.value.sval; }
   void SetSigned(int64_t sval) { m_value.value.sval = sval; }
   const char *AsCString() const;

lldb/source/Plugins/SymbolFile/DWARF/DWARFFormValue.cpp

if (!symbol_file)
     return 0;
 
   uint32_t index_size = m_cu->GetAddressByteSize();
   dw_offset_t addr_base = m_cu->GetAddrBase();
   lldb::offset_t offset = addr_base + m_value.value.uval * index_size;
   return symbol_file->get_debug_addr_data().GetMaxU64(&offset, index_size);
 }
 
+bool DWARFFormValue::FormIsReference() const {
+  switch (m_form) {
+  case DW_FORM_ref1:
+  case DW_FORM_ref2:
+  case DW_FORM_ref4:
+  case DW_FORM_ref8:
+  case DW_FORM_ref_udata:
+  case DW_FORM_ref_addr:
+    return true;
+    break;
+
+  // It cannot be resolved by the Reference() method.
+  case DW_FORM_ref_sig8:
+  default:
+    break;
+  }
+  return false;
+}
+
 uint64_t DWARFFormValue::Reference() const {
   uint64_t die_offset = m_value.value.uval;
   switch (m_form) {
   case DW_FORM_ref1:
   case DW_FORM_ref2:
   case DW_FORM_ref4:
   case DW_FORM_ref8:
   case DW_FORM_ref_udata: