[libc] proxy header for threads.h

[nickdesaulniers]

#89329 (comment)

[nickdesaulniers]

To flush out this issue description more, we'd like to:

1. create libc/hdr/types/cnd_t.h which based on LIBC_FULL_BUILD either

• includes include/llvm-libc-types/cnd_t.h OR
• includes <threads.h>

2. create libc/hdr/threads_macros.h which based on LIBC_FULL_BUILD either

• defines thrd_error and thrd_success OR
• includes <threads.h>

3. replace #include <threads.h> throughout libc/src/ with more specific includes of either libc/hdr/threads_macros.h, or libc/hdr/types/cnd_t.h or other threads related headers (following the same pattern).
4. clean up the TODO left in src/threads/linux/cnd_broadcast.cpp

[lkartik2212]

@nickdesaulniers is there any good-first-issue for me that is available for this open issue?

[nickdesaulniers]

Hi @lkartik2212 we would love your help on this issue. I'll reassign it to you. Please reach out if you have questions, get stuck, or no longer plan to work on this.