Linker: Rewrite dwarfdump checks from r233164

Rewrite the checks from r233164 that I temporarily disabled in r233165.

It turns out that the line-tables only debug info we emit from `llc` is
(intentionally) different on Linux than on Darwin.  r218129 started
skipping emission of subprograms with no inlined subroutines, and
r218702 was a spiritual revert of that behaviour for Darwin.

I think we can still test this in a platform-neutral way.

  - Stop checking for the possibly missing `DW_TAG_subprogram` defining
    the debug info for the real version of `@foo`.
  - Start checking the line tables, ensuring that the right debug info
    was used to generate them (grabbing `DW_AT_low_pc` from the compile
    unit).
  - I changed up the line numbers used in the "weak" version so it's
    easier to follow.

This should hopefully finish off PR22792.

llvm-svn: 233207