[llvm] [Support] Add LLVM_WINDOWS_AVOID_SHELL32_APIS for shell32-free path lookups (PR #209448)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

Thank you for working on this, that's an interesting point I don't think we had in mind when adding these APIs.

I've added some other reviewers with a Windows background, but one question I have is: how often are the environment variables going to resolve to a different path from the shell APIs? e.g., should we maybe consider dropping the shell APIs entirely and only using the environment variables? Because I worry that we're not going to test both configurations to the same degree, so we might be setting some users up for "it works on this Clang but not that Clang" kind of hard-to-debug situations.

https://github.com/llvm/llvm-project/pull/209448