[llvm][support] Add LDBG macro. (#143704)
Add macro that mirror a common usage of logging to output .This makes it easy to have
streaming log like behavior while still using the base debug logging.
I also wanted to avoid inventing a full logging library here while enabling others to change the sink without too much pain, so put it in its own header (this also avoids making Debug depend on raw_ostream beyond forward reference). The should allow a consistent dev experience without fixing the sink too much.
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Co-authored-by: Mehdi Amini <joker.eph@gmail.com>
