[llvm] [CodeView] Read and dump LF_ALIAS (PR #218243)

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Nerixyz wrote:

> Interesting. It sounds like there's a "PDB schema change" coming, if that's the right word for it.

Technically, `LF_ALIAS` existed for some time now as it's also in the original microsoft-pdb repo, but no one used it. Not sure what got it started in this case.

> I seem to recall that Symbol UDTs were a major bottleneck for PDB writing, so it makes sense that Microsoft would seek to shift that information from the symbol record stream over to the type record stream, where it gets merged more efficiently.
> 
> It then just becomes a question of figuring out which typedefs are active in which translation unit, and I don't know how they'll do it. I guess that's a relevant question for LLDB.

LLDB has only looked at the symbol UDTs when searching types. That seems to be what the DIA SDK does today too. I'm hoping to change that. Having this in the type stream also means that we know which values use a typedef - before we only knew that a typedef existed.

https://github.com/llvm/llvm-project/pull/218243