[RemoteAST] Using module names specified by @_originallyDefinedIn for top-level decls marked as so

This should allow runtime de-mangling of those moved symbols.

Author: nkcsgexi

lib/AST/Decl.cpp

@@ -577,8 +577,12 @@ Result->X = SM.getLocFromExternalSource(Locs->SourceFilePath, Locs->X.Line,   \
 }
 
 StringRef Decl::getAlternateModuleName() const {
-  if (auto *OD = Attrs.getAttribute(DeclAttrKind::DAK_OriginallyDefinedIn)) {
-    return static_cast<const OriginallyDefinedInAttr*>(OD)->OriginalModuleName;
+  for (auto *Att: Attrs) {
+    if (auto *OD = dyn_cast<OriginallyDefinedInAttr>(Att)) {
+      if (OD->isActivePlatform(getASTContext())) {
+        return OD->OriginalModuleName;
+      }
+    }
   }
   for (auto *DC = getDeclContext(); DC; DC = DC->getParent()) {
     if (auto decl = DC->getAsDecl()) {

lib/IRGen/GenDecl.cpp

@@ -788,6 +788,17 @@ IRGenModule::getAddrOfContextDescriptorForParent(DeclContext *parent,
     LLVM_FALLTHROUGH;
 
   case DeclContextKind::Module:
+    if (auto *D = ofChild->getAsDecl()) {
+      // If the top-level decl has been marked as moved from another module,
+      // using @_originallyDefinedIn, we should emit the original module as
+      // the context because all run-time names of this decl are based on the
+      // original module name.
+      auto OriginalModule = D->getAlternateModuleName();
+      if (!OriginalModule.empty()) {
+        return {getAddrOfOriginalModuleContextDescriptor(OriginalModule),
+          ConstantReference::Direct};
+      }
+    }
     return {getAddrOfModuleContextDescriptor(cast<ModuleDecl>(parent)),
             ConstantReference::Direct};
   }

lib/IRGen/GenMeta.cpp

@@ -1908,6 +1908,13 @@ IRGenModule::getAddrOfAnonymousContextDescriptor(
     [&]{ AnonymousContextDescriptorBuilder(*this, DC).emit(); });
 }
 
+llvm::Constant *
+IRGenModule::getAddrOfOriginalModuleContextDescriptor(StringRef Name) {
+  return getAddrOfModuleContextDescriptor(OriginalModules.insert({Name,
+    ModuleDecl::create(Context.getIdentifier(Name), Context)})
+                                          .first->getValue());
+}
+
 static void emitInitializeFieldOffsetVector(IRGenFunction &IGF,
                                             SILType T,
                                             llvm::Value *metadata,

lib/IRGen/IRGenModule.h

@@ -510,6 +510,7 @@ class IRGenModule {
   ModuleDecl *ClangImporterModule = nullptr;
   SourceFile *CurSourceFile = nullptr;
 
+  llvm::StringMap<ModuleDecl*> OriginalModules;
   llvm::SmallString<128> OutputFilename;
   llvm::SmallString<128> MainInputFilenameForDebugInfo;
 
@@ -1340,6 +1341,7 @@ private:                            \
                                       ConstantInit definition = ConstantInit());
   llvm::Constant *getAddrOfObjCModuleContextDescriptor();
   llvm::Constant *getAddrOfClangImporterModuleContextDescriptor();
+  llvm::Constant *getAddrOfOriginalModuleContextDescriptor(StringRef Name);
   ConstantReference getAddrOfParentContextDescriptor(DeclContext *from,
                                                      bool fromAnonymousContext);
   ConstantReference getAddrOfContextDescriptorForParent(DeclContext *parent,