[LTO] Fix -save-temps in case -o is not specified.

Currently we create a file called .lto.bc. In UNIX,
ls(1) by default doesn't show up files starting with
a dot, as they're (only by convention) hidden.
This makes the output of -save-temps a little bit
hard to find. Use "a.out.lto.bc" instead if the
output file is not specified.

While here, change an existing -save-temps test to
exercise this more interesting behaviour.

llvm-svn: 265254