[Driver] Adapt Linux::GCCVersion::Parse to match GCC 5 installations

Summary:
Some GCC 5 installations store the libstdc++ includes and GCC-specific files in paths without 
the minor part of the version number, such as

  /usr/include/c++/5
  /usr/lib64/gcc/x86_64-suse-linux/5

Reviewers: cfe-commits, thiagomacieira, jroelofs

Subscribers: tinti, jroelofs

Differential Revision: http://reviews.llvm.org/D14727

llvm-svn: 273012