[lldb][ObjC] Fix method list entry offset calculation (llvm#115571)

The `relative_list_list_entry_t` offset field in the Objective-C runtime
is of type `int64_t`. There are cases where these offsets are negative
values. For negative offsets, LLDB would currently incorrectly
zero-extend the offset (dropping the fact that the offset was negative),
instead producing large offsets that, when added to the
`m_baseMethods_ptr` result in addresses that had their upper bits set
(e.g., `0x00017ff81b3241b0`). We then would try to `GetMethodList` from
such an address but fail to read it (because it's an invalid address).
This would manifest in Objective-C decls not getting completed correctly
(and formatters not working). We noticed this in CI failures on our
Intel bots. This happened to work fine on arm64 because we strip the
upper bits when calling `ClassDescriptorV2::method_list_t::Read` using
the `FixCodeAddress` ABI plugin API (which doesn't do that on Intel).

The fix is to sign-extend the offset calculation.

Example failure before this patch:
```
======================================================================                                                         
FAIL: test_break_dwarf (TestRuntimeTypes.RuntimeTypesTestCase) 
   Test setting objc breakpoints using '_regexp-break' and 'breakpoint set'.                                                   
----------------------------------------------------------------------                                                         
Traceback (most recent call last):                                                                                             
  File "/Users/michaelbuch/Git/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1769, in test_method        
    return attrvalue(self)                                                                                                                                                                                                                                    
  File "/Users/michaelbuch/Git/llvm-project/lldb/test/API/lang/objc/foundation/TestRuntimeTypes.py", line 48, in test_break                                                                                                                                   
    self.expect(                                                                                                                                                                                                                                              
  File "/Users/michaelbuch/Git/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 2370, in expect                                                                                                                                            
    self.runCmd(                                                                                                               
  File "/Users/michaelbuch/Git/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1000, in runCmd             
    self.assertTrue(self.res.Succeeded(), msg + output) 
AssertionError: False is not true : Got a valid type                                                                           
Error output:                                                                                                                                                                                                                                                 
error: <user expression 1>:1:11: no known method '+stringWithCString:encoding:'; cast the message send to the method's return type         
    1 | [NSString stringWithCString:"foo" encoding:1]                                                                                                                                                                                                         
      | ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                                          
                                                               
Config=x86_64-/Users/michaelbuch/Git/lldb-build-main-no-modules/bin/clang                                                      
----------------------------------------------------------------------      ```

Author: Michael137

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp

@@ -394,7 +394,7 @@ bool ClassDescriptorV2::relative_list_entry_t::Read(Process *process,
   lldb::offset_t cursor = 0;
   uint64_t raw_entry = extractor.GetU64_unchecked(&cursor);
   m_image_index = raw_entry & 0xFFFF;
-  m_list_offset = (int64_t)(raw_entry >> 16);
+  m_list_offset = llvm::SignExtend64<48>(raw_entry >> 16);
   return true;
 }