[lld] [LLD][ELF][AArch64] Add BTI Aware long branch thunks (PR #108989)

[Peter Smith via llvm-commits]

================
@@ -2326,6 +2334,21 @@ bool ThunkCreator::createThunks(uint32_t pass,
                 ts = getISDThunkSec(os, isec, isd, rel, src);
               ts->addThunk(t);
               thunks[t->getThunkTargetSym()] = t;
+
+              // When indirect branches are restricted, such as AArch64 BTI
+              // Thunks may need to target a linker generated landing pad
+              // instead of the target.
+              if (t->needsSyntheticLandingPad()) {
+                Thunk *lpt;
+                auto &dr = cast<Defined>(t->destination);
+                std::tie(lpt, isNew) = getSyntheticLandingPad(dr, t->addend);
+                if (isNew) {
+                  InputSection *targetsec = dyn_cast<InputSection>(dr.section);
----------------
smithp35 wrote:

I've used cast<>

The code in isAArch64BTILandingPad has a test
```
if (!isa_and_nonnull<InputSection>(d->section))
```
so `t->needsSyntheticLandingPad()` should return false in those cases.

https://github.com/llvm/llvm-project/pull/108989