[ELF] - Fix error reporting with --strip-debug/--strip-all.

Currently LLD tries to use information about functions and variables location
taking it from debug sections. When --strip-* is given we discard such sections
and that breaks error reporting.
Patch stops discarding such sections and just removes them from InputSections list.

Differential revision: https://reviews.llvm.org/D39550

llvm-svn: 317405