[Driver][SYCL] Improve tool name specification for AOT tools on Windows

[mdtoguchi]

For Windows, we need to be sure to search for the full executable name
as opposed to just the case name for the AOT tools (aoc, ocloc, opencl-aot).
There can be instances of directory names of the same name, causing the
wrong 'binary' to be called.

[AGindinson]

Could Driver::GetProgramPath() be adjusted instead? This would probably scale better, and would involve non-AOT tools as well (or is there a reason why such a case can't fire for the latter?)

[mdtoguchi]

Could Driver::GetProgramPath() be adjusted instead? This would probably scale better, and would involve non-AOT tools as well (or is there a reason why such a case can't fire for the latter?)

The issue I see with modifying GetProgramPath is that the user may not want .exe added to their file (maybe there is no .exe variant available on Windows).

Edit: I may be able to simplify in GetProgramPath instead and make sure that whatever is being searched for isn't a directory.

[mdtoguchi]

@AGindinson, I took a look, and the behaviors cannot be simply modified at the driver level. I would like to keep the changes as is.

[mdtoguchi]

/summary:run

[AGindinson]

I've tried finding something in llvm::sys::fs/::path that would give a full path to the executable and/or extract its actual extension, but failed miserably :)
Also, the Linux failure in a SYCL RT test doesn't seem related to the patch.