[SandboxIR] Remove tight-coupling with LLVM's SwitchInst::CaseHandle (#167093)
SandboxIR's SwitchInst CaseHandle was relying on LLVM IR's
SwitchInst::CaseHandleImpl template, which may call private functions of
SandboxIR's SwitchInst. This creates a dependency cycle which is against
the design principles of Sandbox IR.
The issue was exposed by:
https://github.com/llvm/llvm-project/pull/166842 Thanks to @aengelke for
raising the issue.