[DWARF] Adjust warning condition for .dwo sections with relocations

D106624 added a .dwo warning (when there are relocations) that may fire for
non-debug sections, e.g. `.rodata.dwo` when there is a data symbol foo in
-fdata-sections mode. Adjust it to only warn for .debug sections.

While here, change the diagnostic to be more conventional
https://llvm.org/docs/CodingStandards.html#error-and-warning-messages and use
the relocated section name instead of the relocation section name.

This change does not handle `.zdebug` (support was mostly removed from LLVM) or
`__debug` (Mach-O, no DWO support).

Reviewed By: ayermolo, HaohaiWen

Differential Revision: https://reviews.llvm.org/D153602