[ELF] Don't consider SHF_ALLOC ".debug*" sections debug sections

Fixes PR48071

* The Rust compiler produces SHF_ALLOC `.debug_gdb_scripts` (which
  normally does not have the flag)
* `.debug_gdb_scripts` sections are removed from `inputSections` due
  to --strip-debug/--strip-all
* When processing --gc-sections, pieces of a SHF_MERGE section can be
  marked live separately

`=>` segfault when marking liveness of a `.debug_gdb_scripts` which
is not split into pieces (because it is not in `inputSections`)

This patch circumvents the problem by not treating SHF_ALLOC
".debug*" as debug sections (to prevent --strip-debug's stripping)
(which is still useful on its own).

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D91291