[llvm] [LLVM] Create `lf_alias` nodes for `typedef` and `using` (PR #153936)

[Reid Kleckner via llvm-commits]

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rnk wrote:

+1, right, we mostly need(ed) the binaries to bootstrap file parsing. Now that the tools can read and write these records, the cost of additional binary test inputs outweighs the increase in confidence we get from validating that our tools can read one additional codeview record from MSVC outputs.

Another way of looking at this is that we committed binary files as test inputs in a pre-xz-attack world, and now binary files are one of the biggest negatives on the LLVM project OpenSSF security scorecard:
https://securityscorecards.dev/viewer/?uri=github.com/llvm/llvm-project

So, things have actually changed, standards have increased, and what worked yesterday won't fly today. 😦 

I want to unblock and enable contributions, but I don't want to make the problem worse at this point. I'm sure this feels unfair, but I hope that context helps explain the shift. In an ideal world, we'd do the work to delete the binary inputs by YAML-ifying the parts of the tests that we need to and replacing the rest using other strategies.

https://github.com/llvm/llvm-project/pull/153936