[Driver] Add unit tests for Distro detection

[Driver] Add unit tests for Distro detection

Add a set of unit tests for the distro detection code. The tests use an
in-memory virtual filesystems resembling release files for various
distributions supported. All release files are provided (not only the
ones directly used) in order to guarantee that one of the rules will not
mistakenly recognize the distribution incorrectly due to the additional
files (e.g. Ubuntu as Debian).

Differential Revision: https://reviews.llvm.org/D25869

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288062 91177308-0d34-0410-b5e6-96231b3b80d8