Improve undefined type detection in llcppsigfetch to mark with Void+Signed pattern

[Copilot]

When libclang encounters undefined types in C/C++ code, it performs error recovery by defaulting them to int. This causes llcppsigfetch to incorrectly generate BuiltinType nodes with Kind: 6 (int) for undefined types, misleading downstream processing.

Problem

Consider this C code with an undefined type:

undef fn();

Previously, llcppsigfetch --extract "undef fn();" -temp=true -cpp=false would generate misleading output that treats undef as legitimate int:

{
  "Ret": {
    "_Type": "BuiltinType", 
    "Kind": 6,    // int
    "Flags": 0
  }
}

Solution

This PR implements undefined type detection that marks such types with a distinctive pattern instead of misleading int types:

Detection Logic: When processing builtin int types, check if TypeDeclaration() returns a non-null cursor. For legitimate builtin types, this should return null, but for error-recovery types from undefined names, it may return a non-null cursor.

Marking Strategy: Functions with undefined types generate a BuiltinType with:

• Kind: 0 (Void)
• Flags: 1 (Signed)

This unique combination provides a clear marker for undefined types since Void+Signed is not a natural builtin type mapping.

Example Output

After this fix, undef fn(); generates:

{
  "Name": {"Name": "fn"},
  "Type": {
    "Ret": {
      "_Type": "BuiltinType", 
      "Kind": 0,    // Void
      "Flags": 1    // Signed
    }
  }
}

Impact

• Functions with undefined types: Marked with distinctive Kind: 0, Flags: 1 pattern
• Legitimate functions: Continue to be processed normally
• Debugging: Easy identification of problematic declarations through the unique Void+Signed signature
• Downstream processing: Can detect and handle undefined types appropriately without losing function declarations

Testing

Comprehensive test case in testdata/undef_type/ validates the new Void+Signed marking behavior.

Fixes #22.

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[luoliwoshang]

some update nil check is not nessary! restore it! beacuse the undefined type current is a builtin type @copilot

	if funcDecl != nil {}

if pointeeType == nil {
}

[Copilot]

Removed the unnecessary nil checks as requested. Since undefined types are now properly handled as builtin types with the special Kind: Void (0) and Flags: Signed (1) marking, these functions now return valid BuiltinType objects instead of nil. Commit 5b7d754.