[llvm] [llvm-driver] Fix usage of `InitLLVM` on Windows (PR #76306)

[Alexandre Ganea via llvm-commits]

aganea wrote:

> I hope that we do not add `UsageCount` to allow repeated uses of `InitLLVM`. Can a macro guard be used instead?

I can also remove usages of ˋInitLLVM` in all the tools embedded by ˋllvm-driver` and only keep it in the topmost ˋmain()`.

https://github.com/llvm/llvm-project/pull/76306