[ELF] mergeCmp: work around irreflexivity bug

Some tests (e.g. aarch64-feature-pac.s) segfault in libstdc++ _GLIBCXX_DEBUG
builds (enabled by LLVM_ENABLE_EXPENSIVE_CHECKS).

dyn_cast<ThunkSection> is incorrectly true for any SyntheticSection. std::merge
transitively calls mergeCmp(x, x) (due to __glibcxx_requires_irreflexive_pred)
and will segfault in `ta->getTargetInputSection()`. The dyn_cast<ThunkSection>
issue should be eventually fixed properly, bug `a != b` is robust enough for now.

(cherry picked from commit abd9807)

Author: MaskRay

lld/ELF/Relocations.cpp

@@ -1705,7 +1705,8 @@ static bool mergeCmp(const InputSection *a, const InputSection *b) {
   if (a->outSecOff < b->outSecOff)
     return true;
 
-  if (a->outSecOff == b->outSecOff) {
+  // FIXME dyn_cast<ThunkSection> is non-null for any SyntheticSection.
+  if (a->outSecOff == b->outSecOff && a != b) {
     auto *ta = dyn_cast<ThunkSection>(a);
     auto *tb = dyn_cast<ThunkSection>(b);