[Driver] Parse Debian version as integer when possible. NFC

Replace the string matching for /etc/debian_version with split
integer/string matching algorithm. When the file contains 'major.minor'
version number, parse the major version as integer and use a switch
clause to match it. Otherwise, attempt 'codename/sid' matching using
a StringSwitch.

Differential Revision: https://reviews.llvm.org/D25696

llvm-svn: 284770