[AST] Distributed: Mangle protocol requirement accessors with special $ mangling

This way distributed thunks could be formed in contexts that don't
know what concrete actor they'd be dispatched to on the other side
i.e. `distributed` members in protocol extensions should refer to
protocol requirement accessors.

Author: xedin

lib/AST/ASTMangler.cpp

@@ -3935,22 +3935,21 @@ void ASTMangler::appendDistributedThunk(
          "distributed thunk to mangle must be function decl");
   assert(thunk->getContextKind() == DeclContextKind::AbstractFunctionDecl);
 
-  auto inProtocolExtensionMangleAsReference =
+  auto referenceInProtocolContextOrRequirement =
       [&thunk, asReference]() -> ProtocolDecl * {
-    if (!asReference) {
-      return nullptr;
-    }
+    auto *DC = thunk->getDeclContext();
+    if (!asReference)
+      return dyn_cast_or_null<ProtocolDecl>(DC);
 
-    if (auto extension = dyn_cast<ExtensionDecl>(thunk->getDeclContext())) {
+    if (auto extension = dyn_cast<ExtensionDecl>(DC))
       return dyn_cast_or_null<ProtocolDecl>(extension->getExtendedNominal());
-    }
+
     return nullptr;
   };
 
-  if (auto type = inProtocolExtensionMangleAsReference()) {
-    appendContext(type->getDeclContext(), thunk->getAlternateModuleName());
-    auto baseName = type->getBaseName();
-    appendIdentifier(Twine("$", baseName.getIdentifier().str()).str());
+  if (auto *P = referenceInProtocolContextOrRequirement()) {
+    appendContext(P->getDeclContext(), thunk->getAlternateModuleName());
+    appendIdentifier(Twine("$", P->getNameStr()).str());
     appendOperator("C"); // necessary for roundtrip, though we don't use it
   } else {
     appendContextOf(thunk);