[ELF][test] Improve and reorganize another set of tests

Add file-level comments
Replace trivial Input/*.s with echo ... | llvm-mc
Delete insignificant addresses to make them more tolerant to layout changes
Simplify test output

Merge merge-section-types.s into compatible-section-types.s and add a missed case
Merge gnu-ifunc-gotpcrel.s (added in D19517) into gnu-ifunc-dso.s (added in D35119) and add missed cases
Delete typed-undef.s - covered by executable-undefined-ignoreall.s
Delete emit-relocs-shared.s - covered by emit-relocs-merge.s

Replace copy-rel-pie.s and copy-rel-pie2.s with canonical-plt-pcrel.s, canonical-plt-symbolic.s and copy-rel.s:
add -no-pie cases.
add a case that a canonical PLT can be created for STT_GNU_IFUNC. The logic in Symbols.h was untested:

  // ctor of SharedSymbol
  if (this->type == llvm::ELF::STT_GNU_IFUNC)
    this->type = llvm::ELF::STT_FUNC;

llvm-svn: 371361