Revert r77396.

Original commit message:

    On "Unix", if Program::FindProgramByName is given a name containing
    slashes, just go with it, regardless of whether it looks like it will
    be executable. This follows the behavior of sh(1) more closely.

It's better when behaviour is consistent between platforms. This change also
makes FindExecutable() behave as expected on unix-likes (before this commit, it
used to always succeed).

llvm-svn: 117582