IRGen: create InternalWeakODR named IRLinkage (NFC)

Create a new named IRLinkage type: `InternalWeakODR`.  This just gives
the IRLinkage a semantic name rather than the computed value.  NFC.

Author: compnerd

include/swift/IRGen/Linking.h

@@ -1043,6 +1043,7 @@ struct IRLinkage {
   llvm::GlobalValue::DLLStorageClassTypes DLLStorage;
 
   static const IRLinkage InternalLinkOnceODR;
+  static const IRLinkage InternalWeakODR;
   static const IRLinkage Internal;
 };
 

lib/IRGen/IRGenModule.cpp

@@ -975,10 +975,7 @@ void IRGenModule::addLinkLibrary(const LinkLibrary &linkLib) {
                                           /*isConstant=*/true,
                                           llvm::GlobalValue::WeakODRLinkage,
                                           ForceImportThunk, buf.str());
-      ApplyIRLinkage({llvm::GlobalValue::WeakODRLinkage,
-                      llvm::GlobalValue::HiddenVisibility,
-                      llvm::GlobalValue::DefaultStorageClass})
-          .to(ref);
+      ApplyIRLinkage(IRLinkage::InternalWeakODR).to(ref);
       auto casted = llvm::ConstantExpr::getBitCast(ref, Int8PtrTy);
       LLVMUsed.push_back(casted);
     }

lib/IRGen/Linking.cpp

@@ -37,6 +37,12 @@ const IRLinkage IRLinkage::InternalLinkOnceODR = {
   llvm::GlobalValue::DefaultStorageClass,
 };
 
+const IRLinkage IRLinkage::InternalWeakODR = {
+  llvm::GlobalValue::WeakODRLinkage,
+  llvm::GlobalValue::HiddenVisibility,
+  llvm::GlobalValue::DefaultStorageClass,
+};
+
 const IRLinkage IRLinkage::Internal = {
   llvm::GlobalValue::InternalLinkage,
   llvm::GlobalValue::DefaultVisibility,