IRGen: create new External{Im,Ex}port named linkages

Create two new semantic names: `ExternalImport` and `ExternalExport`.
These are for symbols which are either imported from an external module
or exported for consumption by external modules.

Author: compnerd

include/swift/IRGen/Linking.h

@@ -1045,6 +1045,9 @@ struct IRLinkage {
   static const IRLinkage InternalLinkOnceODR;
   static const IRLinkage InternalWeakODR;
   static const IRLinkage Internal;
+
+  static const IRLinkage ExternalImport;
+  static const IRLinkage ExternalExport;
 };
 
 class ApplyIRLinkage {

lib/IRGen/IRGenModule.cpp

@@ -960,9 +960,7 @@ void IRGenModule::addLinkLibrary(const LinkLibrary &linkLib) {
     encodeForceLoadSymbolName(buf, linkLib.getName());
     auto ForceImportThunk =
         Module.getOrInsertFunction(buf, llvm::FunctionType::get(VoidTy, false));
-    ApplyIRLinkage({llvm::GlobalValue::ExternalLinkage,
-                    llvm::GlobalValue::DefaultVisibility,
-                    llvm::GlobalValue::DLLImportStorageClass})
+    ApplyIRLinkage(IRLinkage::ExternalImport)
         .to(cast<llvm::GlobalValue>(ForceImportThunk));
 
     buf += "_$";
@@ -1084,10 +1082,7 @@ void IRGenModule::emitAutolinkInfo() {
         llvm::Function::Create(llvm::FunctionType::get(VoidTy, false),
                                llvm::GlobalValue::ExternalLinkage, buf,
                                &Module);
-    ApplyIRLinkage({llvm::GlobalValue::ExternalLinkage,
-                    llvm::GlobalValue::DefaultVisibility,
-                    llvm::GlobalValue::DLLExportStorageClass})
-        .to(ForceImportThunk);
+    ApplyIRLinkage(IRLinkage::ExternalExport).to(ForceImportThunk);
 
     auto BB = llvm::BasicBlock::Create(getLLVMContext(), "", ForceImportThunk);
     llvm::IRBuilder<> IRB(BB);

lib/IRGen/Linking.cpp

@@ -49,6 +49,18 @@ const IRLinkage IRLinkage::Internal = {
   llvm::GlobalValue::DefaultStorageClass,
 };
 
+const IRLinkage IRLinkage::ExternalImport = {
+  llvm::GlobalValue::ExternalLinkage,
+  llvm::GlobalValue::DefaultVisibility,
+  llvm::GlobalValue::DLLImportStorageClass,
+};
+
+const IRLinkage IRLinkage::ExternalExport = {
+  llvm::GlobalValue::ExternalLinkage,
+  llvm::GlobalValue::DefaultVisibility,
+  llvm::GlobalValue::DLLExportStorageClass,
+};
+
 bool swift::irgen::useDllStorage(const llvm::Triple &triple) {
   return triple.isOSBinFormatCOFF() && !triple.isOSCygMing();
 }