Do not pass -pie flag to linker if -shared specified.  This matches
the gcc driver and makes it possible to add -pie to $CC or similar and
have it apply in the right places.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172753 91177308-0d34-0410-b5e6-96231b3b80d8