[Driver] Refactor DetectDistro() parameters to take VFS ref only. NFC

Refactor the DetectDistro() function to take a single vfs::FileSystem
reference only, instead of Driver and llvm::Triple::ArchType.
The ArchType parameter was not used anyway, and Driver was only used to
obtain the VFS.

Aside to making the API simpler and more transparent, it makes it
easier to add unit tests for the function in the future -- since
the tests would need only to provide an appropriate VFS.

Differential Revision: https://reviews.llvm.org/D25819

llvm-svn: 284774