[clang] [clang][DebugInfo] Rename _vtable$ to __clang_vtable (PR #183617)

[Carlos Alberto Enciso via cfe-commits]

https://github.com/CarlosAlbertoEnciso approved this pull request.

Nice. Now using a simple test like
```
// test.cpp
int __clang_vtable;
```
and
```
clang -c test.cpp -Wreserved-identifier
test.cpp:2:5: warning: identifier '__clang_vtable' is reserved because it starts with '__' [-Wreserved-identifier]
    2 | int __clang_vtable;
      |     ^
1 warning generated.
```
May add a simple test case to indicate that now that name is a Clang reserved identifier.

https://github.com/llvm/llvm-project/pull/183617