IWYU: Add more workaround mappings for internal bits
Extend our hack section of mappings to work around IWYU incorrectly requiring:
- bits/stdint-uintn.h for stdint.h
- bits/stdint-intn.h for stdint.h
- bits/types/mbstate_t.h for wchar.h
Extend our hack section of mappings to work around IWYU incorrectly requiring: