refs: forbid clang to complain about unreachable code

refs: forbid clang to complain about unreachable code

When `NO_SYMLINK_HEAD` is defined, `create_ref_symlink()` is hard-coded
as `(-1)`, and as a consequence the condition `!create_ref_symlink()`
always evaluates to false, rendering any code guarded by that condition
unreachable.

Therefore, clang is _technically_ correct when it complains about
unreachable code. It does completely miss the fact that this is okay
because on _other_ platforms, where `NO_SYMLINK_HEAD` is not defined,
the code isn't unreachable at all.

Let's use the same trick as in 82e79c63642c (git-compat-util: add
NOT_CONSTANT macro and use it in atfork_prepare(), 2025-03-17) to
appease clang while at the same time keeping the `-Wunreachable` flag
to potentially find _actually_ unreachable code.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>