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

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

> @compnerd are you comfortable proceeding? For context, in our use case these functions are never actually called, we only need the shell32 import gone.

Mo I'm not. The TmpDir2 migration is fine, the rest I'm not comfortable with. We put the module cache in %%LocalAppData% which can change values.

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