[dotest] Fix compiler version number comparison

dotest's version comparision function is just a lexicographical compare
of the version strings. This is obviously wrong. This patch implements
the comparison using distutils.version.LooseVersion as suggested by
Zachary.

Reviewers: zturner, labath, serge-sans-paille

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D58219

llvm-svn: 354047