Dwarf: use a single line table to generate assembly when .loc is used.

Dwarf: use a single line table to generate assembly when .loc is used.

This is to fix PR15408 where an undefined symbol Lline_table_start1 is used.
Since we do not generate the debug_line section when .loc is used,
Lline_table_start1 is not emitted and we can't refer to it when calculating
at_stmt_list for a compile unit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182344 91177308-0d34-0410-b5e6-96231b3b80d8