Rename LinuxDistro to Distro.

The concept of such a software distribution is not tied to the Linux kernel;
for example Debian GNU/Linux, Debian GNU/Hurd, and Debian GNU/kFreeBSD all
share the same source packages and generally the same user-space configuration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178270 91177308-0d34-0410-b5e6-96231b3b80d8