[Windows] Fix cmd line tokenization of unclosed quotes.

When cl::TokenizeWindowsCommandLine received a command line with an
unterminated double-quoted string at the end, it would discard the
text within that string. That doesn't match the behavior of the
standard Windows C library, which will return the text in the unclosed
quoted string as an argv word.

Fixed, and added extra unit tests in that area.

In some cases (specifically the one in Bugzilla #47579) this could
cause TokenizeWindowsCommandLine to return a zero-length list of
arguments, leading to an array overrun at the call site in
windows::GetCommandLineArguments. Added a check there, for extra
safety: now windows::GetCommandLineArguments will return an error code
instead of failing an assertion.

(This change was written as part of https://reviews.llvm.org/D122914,
but split into a separate commit at the last minute at the code
reviewer's suggestion, because it's fixing an unrelated bug in the
same area. The rest of D122914 will follow in the next commit.)