[DWARF] Make -g with empty assembler source work better.

This was sometimes causing clang or llvm-mc to crash, and in other
cases could emit a bogus DWARF line-table header. I did an interim
patch in r352541; this patch should be a cleaner and more complete
fix, and retains the test.

Addresses PR40538.

Differential Revision: https://reviews.llvm.org/D58750

llvm-svn: 355226