Explanations from the Linux kernel commit 6a6155f664e31c9be43cd:

When the kernel is compiled with Clang, -fsanitize=bounds expands to
-fsanitize=array-bounds and -fsanitize=local-bounds.

Enabling -fsanitize=local-bounds with Clang has the side-effect of
inserting traps.

That's why UBSAN_LOCAL_BOUNDS can enable the 'local-bounds' option
only when UBSAN_TRAP is enabled.