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

[Reid Kleckner via llvm-commits]

rnk wrote:

> Sorry, not to be nitpicky, but generally, GUID (or UUID) refers to a 128 bit ID, which in this case would be the 16 bytes hash. So if we call this `__build_guid`, someone might read only the following 16 bytes, which contains the `RSDS` header and only 12 bytes of actual hash - which isn't what probably was intended.
> 
> So either we have the symbol pointing at the raw hash and call it hash or GUID something like that, or we probably should make it clearer in the name that it actually is a debug directory structure.

No worries, I share your concern, if it's not a real 16 byte GUID, we shouldn't call it one.

To your point, this change should document the expected struct.

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