Add support for separating strings for the split debug info DWARF5
proposal. This leaves the strings in the skeleton die as strp,
but in all dwo files they're accessed now via DW_FORM_GNU_str_index.

Add support for dumping these sections and modify the fission-cu.ll
testcase to have the correct strings and form. Fix a small bug
in the fixed form sizes routine that involved out of array accesses
for the table and add a FIXME in the extractFast routine to fix
this up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171779 91177308-0d34-0410-b5e6-96231b3b80d8
Add more comments to what's going on here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171780 91177308-0d34-0410-b5e6-96231b3b80d8