Revert r362743 "Revert "Revert "Reland D61583 [ELF] Error on relocations to...

Revert r362743 "Revert "Revert "Reland D61583 [ELF] Error on relocations to STT_SECTION symbols if the sections were discarded"""

(In effect, reverting "[ELF] Error on relocations to STT_SECTION symbols if the sections were discarded".)

It caused debug info problems in LibreOffice [1] and Chromium/V8 [2].
Reverting until those can be fixed.

It also reverts r362497 "STT_SECTION symbol should be defined" on .eh_frame, .debug*, .zdebug* and .gcc_except_table"
which was landed as a follow-up to the above.

> With -r or --emit-relocs, we warn `STT_SECTION symbol should be defined`
> on relocations to discarded section symbol. This was added as an error
> in rLLD319404, but was not so effective before D61583 (it turned the
> error to a warning).
>
> Relocations from .eh_frame .debug* .zdebug* .gcc_except_table to
> discarded .text are very common and somewhat expected. Don't warn/error
> on them. As a reference, ld.bfd has a similar logic in
> _bfd_elf_default_action_discarded() to allow these cases.
>
> Delete invalid-undef-section-symbol.test because what it intended to
> check is now covered by the updated comdat-discarded-reloc.s
>
> Delete relocatable-eh-frame.s because we allow relocations from
> .eh_frame as a special case now.

And finally it reverts r362218 "[ELF] Replace a dead test in getSymVA() with assert()"
as that also depended on the main change reverted here.

> Symbols relative to discarded comdat sections are Undefined instead of
> Defined now (after D59649 and D61583). The `== &InputSection::Discarded`
> test becomes dead. I cannot find a test related to this behavior.

 [1] http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190603/659848.html
 [2] https://bugs.chromium.org/p/chromium/issues/detail?id=978067

llvm-svn: 364321