[ORC] Remove EDU from dependants list of dependencies before destroying.

Dependant lists hold raw pointers back to EDUs that depend on them. We need to
remove these entries before destroying the EDU or we'll be left with a dangling
reference that can result in use-after-free bugs.

No testcase: This has only been observed in multi-threaded setups that
reproduce the issue inconsistently.

rdar://135403614

Author: lhames

llvm/lib/ExecutionEngine/Orc/Core.cpp

@@ -3607,6 +3607,21 @@ ExecutionSession::IL_failSymbols(JITDylib &JD,
       assert(MI.DefiningEDU->Symbols.count(NonOwningSymbolStringPtr(Name)) &&
              "Symbol does not appear in its DefiningEDU");
       MI.DefiningEDU->Symbols.erase(NonOwningSymbolStringPtr(Name));
+
+      // Remove this EDU from the dependants lists of its dependencies.
+      for (auto &[DepJD, DepSyms] : MI.DefiningEDU->Dependencies) {
+        for (auto DepSym : DepSyms) {
+          assert(DepJD->Symbols.count(SymbolStringPtr(DepSym)) &&
+                 "DepSym not in DepJD");
+          assert(DepJD->MaterializingInfos.count(SymbolStringPtr(DepSym)) &&
+                 "DepSym has not MaterializingInfo");
+          auto &SymMI = DepJD->MaterializingInfos[SymbolStringPtr(DepSym)];
+          assert(SymMI.DependantEDUs.count(MI.DefiningEDU.get()) &&
+                 "DefiningEDU missing from DependantEDUs list of dependency");
+          SymMI.DependantEDUs.erase(MI.DefiningEDU.get());
+        }
+      }
+
       MI.DefiningEDU = nullptr;
     } else {
       // Otherwise if there are any EDUs waiting on this symbol then move