[ELF] Error if --strip-all and --emit-relocs are used together

--strip-all suppresses the creation of in.symtab
This can cause a null pointer dereference in OutputSection::finalize()

  // --emit-relocs => copyRelocs is true
  if (!config->copyRelocs || (type != SHT_RELA && type != SHT_REL))
    return;
  ...
  link = in.symTab->getParent()->sectionIndex; // in.symTab is null

Let's just disallow the combination. In some cases the combination can
cause GNU linkers to fail:

* ld.bfd: final link failed: invalid operation
* gold: internal error in set_no_output_symtab_entry, at ../../gold/object.h:1814

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D66704

llvm-svn: 369878