[lld] [lld][WebAssembly] Report undefined symbols by default -shared/-pie builds (PR #75242)

[Derek Schuff via llvm-commits]

================
@@ -305,13 +306,88 @@ static bool shouldReplace(const Symbol *existing, InputFile *newFile,
     return true;
   }
 
+  // Similarly with shared symbols
+  if (existing->isShared()) {
+    LLVM_DEBUG(dbgs() << "replacing existing shared symbol\n");
+    return true;
+  }
+
   // Neither symbol is week. They conflict.
   error("duplicate symbol: " + toString(*existing) + "\n>>> defined in " +
         toString(existing->getFile()) + "\n>>> defined in " +
         toString(newFile));
   return true;
 }
 
+Symbol *SymbolTable::addSharedFunction(StringRef name, uint32_t flags,
+                                       InputFile *file,
+                                       const WasmSignature *sig) {
+  LLVM_DEBUG(dbgs() << "addSharedFunction: " << name << " [" << toString(*sig)
+                    << "]\n");
+  Symbol *s;
+  bool wasInserted;
+  std::tie(s, wasInserted) = insert(name, file);
+
+  auto replaceSym = [&](Symbol *sym) {
+    replaceSymbol<SharedFunctionSymbol>(sym, name, flags, file, sig);
+  };
+
+  if (wasInserted) {
+    replaceSym(s);
+    return s;
+  }
+
+  auto existingFunction = dyn_cast<FunctionSymbol>(s);
+  if (!existingFunction) {
+    reportTypeError(s, file, WASM_SYMBOL_TYPE_FUNCTION);
+    return s;
+  }
+
+  // Shared symbols should never deplace locally-defined ones
----------------
dschuff wrote:

```suggestion
  // Shared symbols should never replace locally-defined ones
```

https://github.com/llvm/llvm-project/pull/75242