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.

llvm-svn: 193089