[libc] Expose the Sign type for shared use (#150083)
This patch exposes the internal `Sign` type for shared use by other LLVM
subprojects, following the pattern established by `FPBits`.
The `FPBits` utility is exposed via `libc/shared/fp_bits.h`. However,
its public interface relies on the `Sign` type for, e.g., creating
signed infinities and returning the sign of a value. Currently, users of
the shared `FPBits` have no way to access the `Sign` type.
Following the existing pattern for sharing `libc` utilities, this patch
adds a new public header `libc/shared/sign.h`. This header simply
includes the internal `src/__support/sign.h` and brings the `Sign` type
into the `shared` namespace.
