[GWP-ASan] Change unreachable -> trap to work around DCE bug.

trapOnAddress is designed to SEGV on a specific address. Unfortunately,
with an IR change, __builtin_unreachable() ends up doing DCE on things
that have side effects, like the load that causes the trap.

Change to __builtin_trap() to avoid the optimisation.

Root cause is still an LLVM bug, and tracked in
https://bugs.llvm.org/show_bug.cgi?id=47480.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D105654