[msan] Unpoison indirect outputs for userspace when...

[msan] Unpoison indirect outputs for userspace when -msan-handle-asm-conservative is specified (#77393)

KMSAN defaults to `msan-handle-asm-conservative`, which inserts
`__msan_instrument_asm_store` calls to unpoison indirect outputs in
inline assembly (e.g. `=m` constraints in source).

```c
unsigned f() {
  unsigned v;
  // __msan_instrument_asm_store unpoisons v before invoking the asm.
  asm("movl $1,%0" : "=m"(v));
  return v;
}
```

Extend the mechanism to userspace, but require explicit
`-mllvm -msan-handle-asm-conservative` for experiments for now.

As

https://docs.kernel.org/dev-tools/kmsan.html#inline-assembly-instrumentation
says, this approach may mask certain errors (an indirect output may not
actually be initialized), but it also helps to avoid a lot of false
positives.

Link: https://github.com/google/sanitizers/issues/192