[llvm] [llvm] Use the VFS to make path absolute (PR #161271)

[Jan Svoboda via llvm-commits]

jansvoboda11 wrote:

This PR was failing in the Windows CI.

The reason is that we used `ExternalFS->makeAbsolute()` and `llvm::sys::path::append()` that don't handle path separators right on Windows.

I fixed this by switching to calling `FS->makeAbsolute(StringRef, SmallVectorImpl<char> &)`, which is explained in more detail here: https://reviews.llvm.org/D71092 or here: https://github.com/llvm/llvm-project/blob/2802ab673e1613bd158bd2a0998c7604001fc7fb/llvm/lib/Support/VirtualFileSystem.cpp#L1400-L1437.

https://github.com/llvm/llvm-project/pull/161271