[llvm] Introduce LDBG_OS() macro as a variant of LDBG() (PR #157194)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

@banach-space : I revamped the grouping, now I start with a section:

```
// ----------------------------------------------------------------------------
// LDBG() implementation
// ----------------------------------------------------------------------------
```

where I also ensure that all the macros are prefixed with LDBG_ now and the suffixes matches with the LDBG_OS_ impl.

Then there is the section for:

```
// ----------------------------------------------------------------------------
// LDBG_OS() implementation
// ----------------------------------------------------------------------------
```

Modeled around the first section for LDBG() implementation.

And finally a section with:
```
// ----------------------------------------------------------------------------
// General Helpers for the implementation above
// ----------------------------------------------------------------------------
```

PTAL.

https://github.com/llvm/llvm-project/pull/157194