[DebugInfo][Docs] Document MIR language debug-info constructs
This patch documents the MIR syntax for a number of things relevant to
- Trailing 'debug-location' metadata that becomes a DebugLoc,
- Variable location metadata for stack slots,
- Syntax for DBG_VALUE metainstructions,
- Syntax for DBG_INSTR_REF, including trailing instruction numbers attached to MIR instructions.
Differential Revision: https://reviews.llvm.org/D89337