[lld] [LLD][COFF] add __lld_buildid symbol. (PR #74652)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> > Can we avoid `lld` in the name and call it just `__buildid`? I'm worried that calling it `__lld_buildid` might discourage other COFF linkers from adopting it.
> 
> That's reasonable. Can we make the name even more descriptive to reduce the risk of symbol conflicts? There are many implementers in the implementer's namespace (`__`). One idea: `__buildid_guid` `__build_guid` (it is a GUID, isn't it?).

IIRC this doesn't point at a pure GUID, but at a whole debug directory which does contain a GUID and some timestamps and stubs for descriptions about the debug info.

https://github.com/llvm/llvm-project/pull/74652