Fix DelegateGenerator on macOS

Commit 785c9df34d ("Restore symbol lookup in child interpreters")
added a DefinitionGenerator to allow symbol lookup in the parent
IncrementalJIT after the upgrade to LLVM 13. It appears that instead
of the unmangled name, we need to lookup already linker mangled names.

This fixes the tests CodeUnloading/AtExit.C and
MultipleInterpreters/MultipleInterpreters.C on macOS, which adds an
underscore during linker mangling. No change on Linux because there
is no additional linker name mangling.

lib/Interpreter/IncrementalJIT.cpp

@@ -586,7 +586,7 @@ IncrementalJIT::IncrementalJIT(
 
 std::unique_ptr<llvm::orc::DefinitionGenerator> IncrementalJIT::getGenerator() {
   return std::make_unique<DelegateGenerator>(
-      [&](StringRef UnmangledName) { return Jit->lookup(UnmangledName); });
+      [&](StringRef Name) { return Jit->lookupLinkerMangled(Name); });
 }
 
 void IncrementalJIT::addModule(Transaction& T) {