Linker: Stop using -gmlt test/Linker/subprogram-linkonce-weak.ll

As dblaikie pointed out, if I stop setting `emissionKind: 2` then the
backend won't do magical things on Linux vs. Darwin.  I had wrongly
assumed that there were stricter requirements on the input if we weren't
in line-tables-only mode, but apparently not.

With that knowledge, clean up this testcase a little more.

  - Set `emissionKind: 1`.
  - Add back checks for the weak version of @foo.
  - Check more robustly that we have the right subprograms by checking
    the `DW_AT_decl_file` and `DW_AT_decl_line` which now show up.
  - Check the line table in isolation (since it's no longer doubling as
    an indirect test for the subprogram of the weak version of @foo).

llvm-svn: 233221