[lldb][NFCI] Rewrite UnwindAssemblyInstEmulation in terms of a CFG visit (#169630)

Currently, UnwindAssemblyInstEmulation visits instructions in the order
in which they appear in a function. This commit makes an NFCI change to
UnwindAssemblyInstEmulation so that it follows the function's CFG:

1. The first instruction is enqueued.
2. While the queue is not empty:
2.1 Visit the instruction in the *back* queue to compute the new unwind
    state.
2.2 Push(+) the next instruction to the *back* of the queue.
2.3 If the instruction is a forward branch with a known branch target,
    push(+) the destination instruction to the *front* of the queue.

(+) Only push if this instruction hasn't been enqueued before.
(+) When pushing an instruction, the current unwind state is attached to
it.

Note that:
* the "next instruction" is pushed to the *back* of the queue,
* a branch target is pushed to the *front* of the queue, and
* we always dequeue from the *back* of the queue.

This means that consecutive instructions are visited one after the
other; this is important to support "conditional blocks" [1] of
instructions (see the line with "if last_condition != new_condition").
This is arguably a very Thumb specific thing, so maybe it shouldn't be
in the generic algorithm; that said, it is already in the code, so we
have to support it.

The main reason this patch is NFCI and not NFC is that, now, the
destination of a forward branch is visited in a slightly different
moment than before. This should not cause any changes in output, as if a
branch destination is reachable through two different paths, any well
behaved compiler will generate the same unwind state in both paths.

The motivation for this patch is to change step 2.2 so that it _only_
pushes the next instruction if the current instruction is not an
unconditional branch / return, and to change step 2.3 so that backwards
branches are also allowed, fixing the bug described by [2].

[1]:
https://developer.arm.com/documentation/dui0473/m/arm-and-thumb-instructions/it
[2]: https://github.com/llvm/llvm-project/pull/168398

Part of a sequence of PRs:
[lldb][NFCI] Rewrite UnwindAssemblyInstEmulation in terms of a CFG visit
#169630
[lldb][NFC] Rename forward_branch_offset to branch_offset in
UnwindAssemblyInstEmulation #169631
[lldb] Add DisassemblerLLVMC::IsBarrier API #169632
[lldb] Handle backwards branches in UnwindAssemblyInstEmulation #169633

commit-id:dce6b515