[libc++] Remove explicit mentions of __need_FOO macros (#119025)
This change has a long history. It was first attempted naively in
https://reviews.llvm.org/D131425, which didn't work because we broke the
ability for code to include e.g. <stdio.h> multiple times and get
different definitions based on the pre-defined macros.
However, in #86843 we managed to simplify <stddef.h> by including the
underlying system header outside of any include guards, which worked.
This patch applies the same simplification we did to <stddef.h> to the
other headers that currently mention __need_FOO macros explicitly.
llvm-project
ce4ac994 - [libc++] Remove explicit mentions of __need_FOO macros (#119025)
