[DWARFv5] Re-enable dumping a line table with no CU.

r323476 added support for DW_FORM_line_strp, and incorrectly made that
depend on having a DWARFUnit available.  We shouldn't be tracking
.debug_line_str in DWARFUnit after all.  After this patch, I can do an
NFC follow up and undo a bunch of the "plumbing" part of r323476.

Differential Revision: https://reviews.llvm.org/D42609

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323691 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed