Prior to this PR, `int`, `signed int`, and `unsigned int`
bitfields are all treated as `unsigned int`.
With this PR, `signed int` will be treated as `signed int`,
and the others remain unsigned.
Since `Type` does not distinguish between `int` and `signed int`,
add an extra `int* SignenessSpecified` param to `ParseTypeSpec`
so we can tell these apart for bit-fields and treat plain `int : N`
as `unsigned int : N` since it is more efficient to zero-extend
than sign-extend.
Fixes#1095
