[UBSan] Consider zero input to __builtin_clz/ctz to be undefined independent of the target.

Previously we checked isCLZForZeroUndef and only added UBSan checks
if it returned true.

The builtin should be considered undefined for 0 regardless of
the target so that code using it is portable. The isCLZForZeroUndef
was only intended to disable optimizations in the middle end and
backend.

See https://discourse.llvm.org/t/should-ubsan-detect-0-input-to-builtin-clz-ctz-regardless-of-target/71060

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D152023