[llvm-driver] Just use argv[0]'s filename for finding tool

Usually we want the stem of argv[0] so something like clang-15
will correctly be identified as clang. For lld however, ld.lld
or ld-link would have a stem of just ld, so we also want to
use the full filename. The bug previously was that we were using
all of argv[0] so if you use a full path that happens to include a tool
name then that could be found first. This was the case in 2 stage
build where the binaries are stored in "tools/clang/stage2-bins/"
so some tools would end up as clang and not their intended tool.