Encapsulate the DWARF string pool in a separate type.

Pulls out some more code from some of the rather monolithic DWARF
classes. Unlike the address table, the string table won't move up into
DwarfDebug - each DWARF file has its own string table (but there can be
only one address table).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207277 91177308-0d34-0410-b5e6-96231b3b80d8