Driver: default to `unsigned int` `wchar_t` for ARM

AAPCS and AAPCS64 mandate that `wchar_t` with `-fno-short-wchar` is an
`unsigned int` rather than a `signed int`.  Ensure that the driver does
not flip the signedness of `wchar_t` for those targets.

Add additional tests to ensure that this does not regress.

llvm-svn: 316858