Make the debug info in some tests more realistic.

Currently lld just parses the .debug_line section assuming that there
is only one compile unit. That assumption is false (PR36793).

I have a patch that changes lld to iterate over the compile units and
parse the portions of the .debug_line they point to (which fixes
PR36793).

A problem is that we will then need a compiler unit pointing to
.debug_line for lld to see it.

It seems like bfd has the same restriction.

This patch updates existing tests to add a minimal compile unit so
that they still work with PR36793 fixed.

llvm-svn: 328215