TDBGen: add a workaround for a workaround for async PWT

When a protocol witness thunk is formed to a serialised protocol
containing an `async` function, the async function pointer to the
conformance needs to be made public due to a SIL Verifier check failure
(reference to a non-fragile function from within a public fragile
function).  Add a stop-gap solution of rolling an extra emission for a
private symbol as a public symbol to avoid the error (the underlying
issue has been open for ~6y and counting as of this commit).

This was identified by swift-package-manager (#64900).

Author: compnerd

lib/IRGen/TBDGen.cpp

@@ -472,9 +472,15 @@ void TBDGenVisitor::addProtocolWitnessThunk(RootProtocolConformance *C,
                                             ValueDecl *requirementDecl) {
   Mangle::ASTMangler Mangler;
 
+  std::string decorated = Mangler.mangleWitnessThunk(C, requirementDecl);
   // FIXME: We should have a SILDeclRef SymbolSource for this.
-  addSymbol(Mangler.mangleWitnessThunk(C, requirementDecl),
-            SymbolSource::forUnknown());
+  addSymbol(decorated, SymbolSource::forUnknown());
+
+  ValueDecl *PWT = C->getWitness(requirementDecl).getDecl();
+  if (fixmeWitnessHasLinkageThatNeedsToBePublic(SILDeclRef(PWT)))
+    if (const auto *AFD = dyn_cast<AbstractFunctionDecl>(PWT))
+      if (AFD->hasAsync())
+        addSymbol(decorated + "Tu", SymbolSource::forUnknown());
 }
 
 void TBDGenVisitor::addFirstFileSymbols() {