[lldb][NFC] Rename forward_branch_offset to branch_offset in UnwindAssemblyInstEmulation (#169631)
This will reduce the diff in subsequent patches
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:5e758a22