`dwarf::TagString()` shouldn't stringify `DW_TAG_lo_user` or
`DW_TAG_hi_user`. These aren't actual tags; they're markers for the
edge of vendor-specific tag regions.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228029 91177308-0d34-0410-b5e6-96231b3b80d8