[libc] limits.h: include compiler limits if not already included

The LLVM-libc build itself will override include paths and prefer it's
own limits.h over the compiler's limits.h. Because we rely on the
compiler limits.h for numerical limits in LLVM-libc it needs to be
include_next:ed if not already included. The other method to work
around this is to define all numeric macros in place.

Signed-off-by: Alfred Persson Forsberg <cat@catcream.org>

Reviewed By: thesamesam

Differential Revision: https://reviews.llvm.org/D158040