[llubi] Add support for undef values (#205602)

Although we are planning to deprecate the undef value, it is still
widely used in the intermediate results of the pipeline, which blocks
the pass bisection. This patch uses `freeze poison` as a refinement of
undef.

Note that the undef value evaluates to different values each time the
user is executed. So it cannot be cached like other constants. A
temporary buffer is introduced to take ownership of these values and
avoid breaking the interface (although this is a bit ugly...). This will
also be used by a follow-up patch for ptrtoint/inttoptr.

From my experience, it is enough for test case reduction of middle-end
miscompilation bugs (there are still counterexamples like
https://github.com/dtcxzyw/llvm-autoreduce/issues/61). However, when
processing backend miscompilation bugs, lli typically uses a garbage
value, so that llvm-reduce may produce an invalid result. I think we may
need to introduce two flags to migrate this issue: one for poisoning
uninitialized memory, and another for diagnosing immediately when
non-deterministic behaviour occurs. This would prevent llvm-reduce from
converting a deterministic repro into a non-deterministic one.

`llvm/test/tools/llubi/unsupported_constant.ll` is also dropped because
it is annoying to pick another unimplemented constant value...