[lld-macho] Fix bug in makeSyntheticInputSection when -dead_strip flag is specified (#86878)

Previously, `makeSyntheticInputSection` would create a new
`ConcatInputSection` without setting `live` explicitly for it. Without
`-dead_strip` this would be OK since `live` would default to `true`.
However, with `-dead_strip`, `live` would default to false, and it would
remain set to `false`.
This hasn't resulted in any issues so far since no code paths that
exposed this issue were present.
However a recent change - ObjC relative method lists
(https://github.com/llvm/llvm-project/pull/86231) exposes this issue by
creating relocations to the `SyntheticInputSection`.
When these relocations are attempted to be written, this ends up with a
crash(assert), since the `SyntheticInputSection` they refer to is marked
as dead (`live` = `false`).

With this change, we set the correct behavior - `live` will always be
`true`. We add a test case that before this change would trigger an
assert in the linker.

1 files changed, 1 insertions, 1 deletions

diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
index 825242f2cc72..755ff270e2f7 100644
--- a/lld/MachO/SymbolTable.cpp
+++ b/lld/MachO/SymbolTable.cpp
@@ -377,7 +377,7 @@ static void handleSectionBoundarySymbol(const Undefined &sym, StringRef segSect,
     // live. Marking the isec live ensures an OutputSection is created that the
     // start/end symbol can refer to.
     assert(sym.isLive());
-    isec->live = true;
+    assert(isec->live);
 
     // This runs after gatherInputSections(), so need to explicitly set parent
     // and add to inputSections.