[DWARFv5] llvm-mc -dwarf-version does not imply -g.

Don't provide the assembler source as the "root file" unless the user
asked to have debug info for the assembler source (with -g).

If the source doesn't provide an explicit ".file 0" then (a) use the
compilation directory as directory #0, and (b) use the file #1 info
for file #0 also.

Differential Revision: https://reviews.llvm.org/D48055

llvm-svn: 334512