Issue Description:

In standard library, {{CChar}} and {{CWideChar}} are mapped to fixed Swift types. But their corresponding C type {{char}} and {{wchar_t}} have varied signs on different platforms. As remarked here:

> Android and Linux IIRC are unsigned char for char (on ARM/AArch64) and Windows and macOS are signed char (on ARM/ARM64). On x86/x86_64, all of them should be signed char. On PPC/PPC64 there is a bug I believe that only Darwin will be treated as signed char when it should mirror the ARM/AArch64 case.

Swift should find a solution where the imported API and stdlib types have matched sign. Or somehow make this a non-issue (by introducing a signless byte type, for example).