DebugInfo: Implement DW_AT_stmt_list for type units

Type units will share the statement list of their defining compile unit.
This is a tradeoff that reduces .o debug info size at the cost of some
linked debug info size (since the contents of those string tables won't
be deduplicated along with the type unit) which seems right for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201445 91177308-0d34-0410-b5e6-96231b3b80d8