executables should be validated before spawning subprocesses

dotest.py script doesn't validate executables passed on the command line
before spawning dozens of subprocesses, all of which fail silently,
leaving an empty results file.

We should validate the lldb and compiler executables on
configuration, aborting when given invalid paths, to prevent numerous,
cryptic, and spurious failures.

<rdar://problem/31117272>

llvm-svn: 298111