Driver: Handle properly calling dsymutil when source input is

preceeded by a linker input flag.
 - <rdar://problem/6757236> clang should make a dSYM when going
   straight from source to binary

 - This still matches gcc, but the right way to solve this would be to
   detect the situation we care about (we are compiling from source
   and linking in one step), instead of looking at the suffix of the
   input file. The Tool doesn't quite have enough information to do
   this yet, however.

 - Also, find the suffix correctly.

llvm-svn: 68417