[libc][stdbit] Typos in type generic macros for `bit_width`, `bit_floor`

[ashfordium]

I think I noticed a potential typo in the stdbit type generic macros for stdc_bit_width and stdc_bit_floor:

#define stdc_bit_width(x)                                                      \
  _Generic((x),                                                                \
      unsigned char: stdc_bit_width_ui, <---                                   \
      unsigned short: stdc_bit_width_us,                                       \
      unsigned: stdc_bit_width_ui,                                             \
      unsigned long: stdc_bit_width_ul,                                        \
      unsigned long long: stdc_bit_width_ull)(x)
#define stdc_bit_floor(x)                                                      \
  _Generic((x),                                                                \
      unsigned char: stdc_bit_floor_ui, <---                                   \
      unsigned short: stdc_bit_floor_us,                                       \
      unsigned: stdc_bit_floor_ui,                                             \
      unsigned long: stdc_bit_floor_ul,                                        \
      unsigned long long: stdc_bit_floor_ull)(x)

I think the assignment exprs for the assocs for unsigned char should refer to the respective *_uc functions.

Seems like this is missed by the tests because we only test the __cplusplus part of the preprocessor conditional.

I will fix these and add tests for both branches of the preprocessor conditional in stdbit-macros.h, let me know if these are actually as expected.

[ashfordium]

Actually, I am not sure of how to write tests for both branches of the preproc cond in stdbit-macros.h using the libc testing infra. Should the tests be new code written in C? Could someone point me in the right direction?

[lntue]

I think to test the other code path, we just need to have a test in C (.c) instead of C++ (.cpp). But I don't think our current testing framework works for C.

You could try to add a C test in test/include with some simple assertion using the libc.src.assert.__assert_fail target. We can extend it to a separate test framework for C tests later.