[LLD][ELF] Prefer placing ThunkSections before non ThunkSections

[peter.smith]

Summary

When there is an inline thunks such as the Mips LA25 and a non inline thunk in the same OutputSection at the same OutSecOff we must make sure that we output the inline thunk last as control may drop through.

At present we haven't implemented any ARM inline thunks yet, and there aren't any non-inline thunks for Mips so this change may not be necessary just yet.

[peter.smith]

I'm abandoning the range thunks implementation that inserts into OutputSections in favour of an implementation that inserts into InputSectionDescription.

ELF/Relocations.cpp

for (auto &KV : NewThunkSections) {
     ThunkEnd = std::remove_if(ThunkBegin, ThunkEnd, [](const ThunkSection *TS) {
       return TS->getSize() == 0;
     });
     if (ThunkBegin == ThunkEnd)
       continue;
 
     // Order Thunks in ascending OutSecOff
     auto ThunkCmp = [](const ThunkSection *A, const ThunkSection *B) {
+      if (A->OutSecOff == B->OutSecOff) {
+        // If we have two ThunkSections at the same address and one has a
+        // specific Target then the specific Target comes last as control may
+        // fall through from the Thunk to the Target
+        if (!A->getTargetInputSection() && B->getTargetInputSection())
+          return true;
+        return false;
+      }
       return A->OutSecOff < B->OutSecOff;
     };
     std::stable_sort(ThunkBegin, ThunkEnd, ThunkCmp);
 
     // Merge sorted vectors of Thunks and InputSections by OutSecOff
     std::vector<InputSection *> Tmp;
     Tmp.reserve(OS->Sections.size() + std::distance(ThunkBegin, ThunkEnd));
     auto MergeCmp = [](const InputSection *A, const InputSection *B) {
       // std::merge requires a strict weak ordering.
       if (A->OutSecOff < B->OutSecOff)
         return true;
-      if (A->OutSecOff == B->OutSecOff)
+      if (A->OutSecOff == B->OutSecOff) {
+        auto *TA = dyn_cast<ThunkSection>(A);
+        auto *TB = dyn_cast<ThunkSection>(B);
         // Check if Thunk is immediately before any specific Target InputSection
         // for example Mips LA25 Thunks.
-        if (auto *TA = dyn_cast<ThunkSection>(A))
-          if (TA && TA->getTargetInputSection() == B)
-            return true;
+        else if (TA && !TB && !TA->getTargetInputSection())
+          // In general place Thunk Sections without specific targets before
+          // non-Thunk Sections
+          return true;
+      }
       return false;
     };
     std::merge(OS->Sections.begin(), OS->Sections.end(), ThunkBegin, ThunkEnd,
                std::back_inserter(Tmp), MergeCmp);
     OS->Sections = std::move(Tmp);
 
     // We need to insert the sections into the linker script input section
     // commands for the correct output order