Driver: Keep track of a separate "install dir", which is the path where clang

Driver: Keep track of a separate "install dir", which is the path where clang
was invoked from (which may not be where the executable itself is).
 - This allows having e.g., /Developer/usr/bin/clang be a symlink to some other
   location, while still making sure the Driver finds 'as', 'ld', etc. relative
   to itself.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@109989 91177308-0d34-0410-b5e6-96231b3b80d8