Driver: hoist the `wchar_t` handling to the driver

Move the logic for determining the `wchar_t` type information into the
driver.  Rather than passing the single bit of information of
`-fshort-wchar` indicate to the frontend the desired type of `wchar_t`
through a new `-cc1` option of `-fwchar-type` and indicate the
signedness through `-f{,no-}signed-wchar`.  This replicates the current
logic which was spread throughout Basic into the
`RenderCharacterOptions`.

Most of the changes to the tests are to ensure that the frontend uses
the correct type.  Add a new test set under `test/Driver/wchar_t.c` to
ensure that we calculate the proper types for the various cases.

llvm-svn: 315126