Fix FindExecutable to use sys::Path::GetMainExecutable instead of

just argv[0]. And remove the code for searching the current
working directory and for searching PATH; the point of FindExecutable
is not to find whatever version of the executable can be found by
searching around, but to find an executable that accompanies the
current executable.

Update the tools to use sys::Program::FindProgramByName when they
want PATH searching.

llvm-svn: 78240