[clang] [clang][NFC] Add export pragmas (PR #175029)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

We currently only have two IWYU pragmas in Clang, but I suspect those additions were missed during code review because we generally don't want tool-specific comments in the tree (this also comes up for comments specific to clang-format, linting tools, etc).

I don't think it's an official policy, more of "this is just how things have always been" stance. But given the lack of these comments in the code base, I think we shouldn't add these (and should remove the existing uses). Alternatively, maybe we should have a community RFC about tool comments to see if the community wants a policy that allows such comments (or explicitly wants to document we don't want those comments).

https://github.com/llvm/llvm-project/pull/175029