[llubi] Add initial support for llubi (#180022)

This patch implements the initial support for upstreaming
[llubi](https://github.com/dtcxzyw/llvm-ub-aware-interpreter). It only
provides the minimal functionality to run a simple main function. I hope
we can focus on the interface design in this PR, rather than trivial
implementations for each instruction.
RFC link:
https://discourse.llvm.org/t/rfc-upstreaming-llvm-ub-aware-interpreter/89645

Excluding the driver `llubi.cpp`, this patch contains three components
for better decoupling:
+ `Value.h/cpp`: Value representation
+ `Context.h/cpp`: Global state management (e.g., memory) and
interpreter configuration
+ `Interpreter.cpp`: The main interpreter loop

Compared to the out-of-tree version, the major differences are listed
below:
+ The interpreter logic always returns the control to its caller, i.e.,
it never calls `exit/abort` when immediate UBs are triggered.
+ `EventHandler` provides an interface to dump the trace. It also allows
callers to inspect the actual value and verify the correctness of
analysis passes (e.g, KnownBits/SCEV).
+ The context is designed to be reentrant. That is, you can call
`runFunction` multiple times. But its usefulness remains in doubt due to
side effects made by previous calls.
+ `runFunction` handles function calls with a loop, instead of calling
itself recursively. This makes it no longer bounded by the stack depth.
+ Uninitialized memory is planned to be approximated by returning random
values each time an uninitialized byte is loaded.