[Driver] Do not lose already detected set of toolchain's multilibs while
iterating over different library path suffixes and different library versions.

To find the most appropriate library for the given command line flags we
iterate over a set of disk paths. Before probe each path the already
detected set of multilibs are cleared. If the set of paths contains
existing paths which do not satisfy command line flags or do not contain
necessary libraries and object files at all we might lose found multilibs.

The patch updates variables which hold detected multilibs if we really find
a new multilib matches command line flags.

The patch reviewed by Jon Roelofs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208523 91177308-0d34-0410-b5e6-96231b3b80d8