[clang] [Clang] Add -Wtrivial-auto-var-init warning for unreachable variables (PR #178318)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114457 .

It looks like computed goto doesn't work, but I guess that's not too surprising.

```
void use(int*);
void switch_precase(int c) {
  void *p = &&INDIRECT;
  goto *p;
    int x;
  INDIRECT:
    use(&x);
  goto *p;
}
```

https://github.com/llvm/llvm-project/pull/178318