[llvm] [CodeView] Match MSVC implicit this records for Visual Studio (PR #214289)

[via llvm-commits]

Nerixyz wrote:

I think this is more of a bug with the Visual Studio debugger. Both LLDB and WinDbg will show the value of `this` when it's available.

---

Long aside: The range for `this` looks slightly off. I'll use the example from above. You can also check it on compiler explorer: https://godbolt.org/z/3caPTnha7

```asm
; foo::foo() [addr = 0001:0032, code size = 24]:
   140001020:	50                   	push   rax
   140001021:	48 89 0c 24          	mov    QWORD PTR [rsp],rcx
   140001025:	48 8b 04 24          	mov    rax,QWORD PTR [rsp]      ;                        ⎤ "Real" range of `this`
   140001029:	c7 00 01 00 00 00    	mov    DWORD PTR [rax],0x1      ; ⎤ DEFRANGE of `this`   │
   14000102f:	c7 40 04 02 00 00 00 	mov    DWORD PTR [rax+0x4],0x2  ; │ [0001:0041,+15)      │
   140001036:	59                   	pop    rcx                      ; │                      ⎦
   140001037:	c3                   	ret                             ; ⎦
```

We say that `this`ranges from `[0x140001029, 0x140001038)`, but you can see in the disassembly, that `[rsp]` is set to the `this` value at `0x140001021` - so one instruction after, we already have the value. Similar for the end: with `pop rcx`, we remove the value from the stack, so at the `ret`, `this` is no longer at `[rcx]`.

https://github.com/llvm/llvm-project/pull/214289