[ELF] Improve sh_info=0 and sh_info>=num_sections diagnostic for SHT_REL/SHT_RELA

PR52408 reported an sh_info=0 instance. I have seen sh_info=0
independently before.

sh_info>=num_sections is probably very rare. Just use one diagnostic for
the two types of errors.

Delete invalid-relocations.test which is covered by invalid/bad-reloc-target.test

Differential Revision: https://reviews.llvm.org/D113466