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

[Tom Tromey via llvm-commits]

tromey wrote:

> But, yeah, I wouldn't mind hearing more about lldb's needs/preferences/hopes/dreams for this feature so we might get a design that's more portable at least between SCE and LLDB. (bonus points if anyone's got GDB's needs in mind - perhaps @tromey might be able to lend us some insight as to how GDB does things and what they might be inclined to use/support to improve this feature area)

For C++, GDB knows the details of the Itanium ABI.  When `set print object` is enabled, it uses these to find the runtime type of an object.  It's somewhat buggy, though, since it too does not understand types local to a function.

For Rust, we added a feature to LLVM to enable this.  In Rust, a "trait object" has a vtable pointer and a pointer to the underlying real object.  To discover the type of this real object, the vtable is emitted like:
```
 <1><2a>: Abbrev Number: 2 (DW_TAG_variable)
    <2b>   DW_AT_name        : (indirect string, offset: 0xda): <std::rt::lang_start::{closure_env#0}<()> as core::ops::function::Fn<()>>::{vtable}
    <2f>   DW_AT_type        : <0x3d>
    <33>   DW_AT_location    : 9 byte block: 3 68 5e 5 0 0 0 0 0 	(DW_OP_addr: 55e68)
 <1><3d>: Abbrev Number: 3 (DW_TAG_structure_type)
    <3e>   DW_AT_containing_type: <0xb5>
    <42>   DW_AT_name        : (indirect string, offset: 0x178): <std::rt::lang_start::{closure_env#0}<()> as core::ops::function::Fn<()>>::{vtable_type}
    <46>   DW_AT_byte_size   : 48
    <47>   DW_AT_alignment   : 8
... members ...
```
That is, the vtable is emitted as a global variable.  It's type describes the vtable itself (of course).  But the vtable type has a `DW_AT_containing_type` that points to the runtime type corresponding to this particular vtable.

I tend to think C++ should do something like this as well.  The reason for this approach is that it makes it simple to go from some C++ object in memory to the runtime type: fetch the vtable pointer, look through the DWARF for the object at this address (which can sometimes be problematic as pointed out earlier), then examine the "containing type" to find the DWARF for the real type.

Existing code for Rust is [here](https://github.com/llvm/llvm-project/blob/main/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp#L1039-L1045).

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