[clang] [llvm] [clang][DebugInfo] Add symbol for debugger with VTable information. (PR #130255)

[Carlos Alberto Enciso via llvm-commits]

CarlosAlbertoEnciso wrote:

As pointed out by @Michael137 

> If we're given the `_vtable$` artificial member, there's nothing useful we can do with it right?
```
0x00000042:   DW_TAG_variable
                DW_AT_specification (0x0000005c "_vtable$")
                DW_AT_alignment     (8)
                DW_AT_location      (DW_OP_addrx 0x1)
                DW_AT_linkage_name  ("_ZTV8CDerived")

0x0000004c:   DW_TAG_structure_type ("CDerived")
                ...
0x0000005c:     DW_TAG_variable
                  DW_AT_name  ("_vtable$")
                  DW_AT_type  (0x00000041 "void *")
                  DW_AT_external    (true)
                  DW_AT_declaration (true)
                  DW_AT_artificial  (true)
                  DW_AT_accessibility     (DW_ACCESS_private)
                  DW_AT_containing_type <0x00000042>  --> VTable global variable
              ...

.debug_addr contents:
Addrs: [
0x0000000000000000
0x0000000000000000  <- DW_OP_addrx 0x1
0x0000000000000000
]
```
And taking a variation of what @tromey described, maybe we can add a `DW_AT_containing_type` pointing back to the global variable, providing 2 ways to use the new vtable information entry:
- Given the `_vtable$` artificial member: use the `DW_AT_containing_type` to find the vtable global variable.
- Given the vtable global variable: use the `DW_AT_specification` to find the object definition.


https://github.com/llvm/llvm-project/pull/130255