[Driver] Expand the executable path in the target create output

Resolve the path in the target create output. This is nice when passing
relative paths to the lldb command line driver.

  $ lldb ./binary
  (lldb) target create "./binary"
  Current executable set to '/absolute/path/to/binary' (x86_64).

This change only affects the target create output and does not change
the debugger's behavior. It doesn't resolve symbolic links so it won't
cause confusing when debugging something like clang++ that's symlinked
to clang.

Differential revision: https://reviews.llvm.org/D65611

llvm-svn: 368182