Fix several bugs in r221220's new program finding code.

In both the Unix and Windows variants, std::getenv was called and the
result passed directly to a function accepting a StringRef. This isn't
OK because it might return a null pointer and that causes the StringRef
constructor to assert (and generally produces crash-prone code if
asserts are disabled). Fix this by independently testing the result as
non-null prior to splitting things.

This in turn uncovered another bug in the Unix variant where it would
infinitely recurse if PATH="", or after this fix if PATH isn't set.
There is no need to recurse at all. Slightly re-arrange the code to make
it clear that we can just fixup the Paths argument based on the
environment if we find anything.

I don't know of a particularly useful way to test these routines in
LLVM. I'll commit a test to Clang that ensures that its driver correctly
handles various settings of PATH. However, I have no idea how to
correctly write a Windows test for the PATHEXT change. Any Windows
developers who could provide such a test, please have at. =D

Many thanks to Nick Lewycky and others for helping debug this. =/ It was
quite nasty for us to track down.