Avoid code duplication in the SEH version of UnwindCursor::getRegisterName. NFC. This requires making Registers_*::getRegisterName static. Differential Revision: https://reviews.llvm.org/D55610 git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@348981 91177308-0d34-0410-b5e6-96231b3b80d8