[DWARF] Fixing a bug in DWARF v5 string offsets tables where the length encoded the contribution

length excluding the table header. Instead it must encode the contribution length minus the length
field itself.

Reviewer: JDevliegehere

Differential Revision: https://reviews.llvm.org/D45922


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332030 91177308-0d34-0410-b5e6-96231b3b80d8