DebugInfo: Hash DW_FORM_GNU_str_index as a string.

Found while adding type safety to the various DWARF enumerations (form,
attribute, tag, etc) that caused Clang to warn on an incompletely
covered switch. Converting the comment to a default/unreachable
uncovered this case of an unsupported form encoding. Seems we were
skipping fission strings entirely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193089 91177308-0d34-0410-b5e6-96231b3b80d8