update ManualDWARFIndex::Index to use std::once (#165896)

Small change to use (what I think is) a better practice -- we were using
the `m_indexed` bool member to make sure we called `Index()` once, but
we should just use `std::once`! This change shouldn't affect
functionality.

This change may also make concurrent access to `Index()` thread-safe,
though the ManualDWARFIndex API isn't completely thread-safe due to
`Decode()`. I'm not sure if ManualDWARFIndex was ever intended to be
thread-safe.

Test Plan:

`ninja check-lldb`

Tested basic debugging workflow of a couple of large projects I had
built. Basically:
```
(lldb) target create <project>
(lldb) b main
(lldb) r
(lldb) step
... 
```

I A/B tested the performance of launching several modules with parallel
module loading and didn't observe any performance regressions.

---------

Co-authored-by: Tom Yang <toyang@fb.com>

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