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

[Reid Kleckner via llvm-commits]

rnk wrote:

I think @AaronBallman has the right idea. We should just check the environment variables all the time.

The main reason you would want to use these APIs to get the true home directory is when you are writing a service that operates on behalf of multiple users, but I think we can assume that even long-running compiler services should have a single user dedicated home directory, and it's most convenient if that directory can be overridden using `USERPROFILE` in the environment. I'll dig a bit more.

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