IRGen: use ApplyIRLinkage more aggressively (NFC)

Use `ApplyIRLinkage` rather than manually applying the DLLStorage.  This
makes it much more apparent as to the desired semantic linkage desired.

Author: compnerd

lib/IRGen/GenDecl.cpp

@@ -1410,9 +1410,8 @@ void IRGenModule::emitVTableStubs() {
 
     if (F.getEffectiveSymbolLinkage() == SILLinkage::Hidden)
       alias->setVisibility(llvm::GlobalValue::HiddenVisibility);
-
-    if (useDllStorage())
-      alias->setDLLStorageClass(llvm::GlobalValue::DLLExportStorageClass);
+    else
+      ApplyIRLinkage(IRLinkage::ExternalExport).to(alias);
   }
 }
 

lib/IRGen/GenFunc.cpp

@@ -77,6 +77,7 @@
 #include "swift/AST/PrettyStackTrace.h"
 #include "swift/AST/SubstitutionMap.h"
 #include "swift/AST/Types.h"
+#include "swift/IRGen/Linking.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/CodeGen/CodeGenABITypes.h"
 #include "llvm/IR/Constants.h"
@@ -1621,9 +1622,8 @@ void irgen::emitBlockHeader(IRGenFunction &IGF,
   auto NSConcreteStackBlock =
       IGF.IGM.getModule()->getOrInsertGlobal("_NSConcreteStackBlock",
                                              IGF.IGM.ObjCClassStructTy);
-  if (IGF.IGM.useDllStorage())
-    cast<llvm::GlobalVariable>(NSConcreteStackBlock)
-        ->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
+  ApplyIRLinkage(IRLinkage::ExternalImport)
+      .to(cast<llvm::GlobalVariable>(NSConcreteStackBlock));
 
   //
   // Set the flags.

lib/IRGen/IRGenModule.cpp

@@ -659,8 +659,8 @@ IRGenModule::createStringConstant(StringRef Str,
       return NAME;                                                             \
     NAME = Module.getOrInsertGlobal(SYM, FullTypeMetadataStructTy);            \
     if (useDllStorage() && !isStandardLibrary())                               \
-      cast<llvm::GlobalVariable>(NAME)->setDLLStorageClass(                    \
-          llvm::GlobalValue::DLLImportStorageClass);                           \
+      ApplyIRLinkage(IRLinkage::ExternalImport)                                \
+          .to(cast<llvm::GlobalVariable>(NAME));                               \
     return NAME;                                                               \
   }
 
@@ -681,9 +681,8 @@ llvm::Constant *IRGenModule::getObjCEmptyCachePtr() {
     // struct objc_cache _objc_empty_cache;
     ObjCEmptyCachePtr = Module.getOrInsertGlobal("_objc_empty_cache",
                                                  OpaquePtrTy->getElementType());
-    if (useDllStorage())
-      cast<llvm::GlobalVariable>(ObjCEmptyCachePtr)
-          ->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
+    ApplyIRLinkage(IRLinkage::ExternalImport)
+        .to(cast<llvm::GlobalVariable>(ObjCEmptyCachePtr));
   } else {
     // FIXME: Remove even the null value per rdar://problem/18801263
     ObjCEmptyCachePtr = llvm::ConstantPointerNull::get(OpaquePtrTy);
@@ -714,9 +713,8 @@ Address IRGenModule::getAddrOfObjCISAMask() {
   assert(TargetInfo.hasISAMasking());
   if (!ObjCISAMaskPtr) {
     ObjCISAMaskPtr = Module.getOrInsertGlobal("swift_isaMask", IntPtrTy);
-    if (useDllStorage())
-      cast<llvm::GlobalVariable>(ObjCISAMaskPtr)
-          ->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
+    ApplyIRLinkage(IRLinkage::ExternalImport)
+        .to(cast<llvm::GlobalVariable>(ObjCISAMaskPtr));
   }
   return Address(ObjCISAMaskPtr, getPointerAlignment());
 }