Fix typo in ArmUnwindInfo::GetUnwindPlan

Summary:
As reported in LLVM bug 41486, the check `(byte1 & 0xf8) == 0xc0` is wrong. We want to check for `11010nnn`,
so the proper value we want to compare against is `0xd0` (`0xc0` would check for the value `11000nnn` which we
already checked for above as described in the bug report).

Reviewers: #lldb, jasonmolenda

Reviewed By: #lldb, jasonmolenda

Subscribers: jasonmolenda, javed.absar, kristof.beyls, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D60655

llvm-svn: 358479