Revert r117582, which reverted r77396. Searching PATH for a string

which contains slashes is inconsistent with the meaning of PATH on
Unix-type platforms, and pretty surprising.

If the user has given a specific path to execute and we can't
execute it, we should fail and say why.  (Apparently the new
posix_spawn code doesn't always say why, but that's a separate
issue.)

llvm-svn: 117596