IRGen: use named IRLinkage a bit more (NFC)

Use the `IRLInkage::InternalLinkOnceODR` linkage rather than computing
that in a couple of sites.  This gives us semantic meaning to the
linkage being applied.  NFC.

lib/IRGen/GenReflection.cpp

@@ -230,10 +230,7 @@ llvm::Constant *IRGenModule::getMangledAssociatedConformance(
                                       llvm::GlobalValue::LinkOnceODRLinkage,
                                       nullptr,
                                       symbolName);
-  ApplyIRLinkage({llvm::GlobalValue::LinkOnceODRLinkage,
-                  llvm::GlobalValue::HiddenVisibility,
-                  llvm::GlobalValue::DefaultStorageClass})
-      .to(var);
+  ApplyIRLinkage(IRLinkage::InternalLinkOnceODR).to(var);
   var->setAlignment(2);
   setTrueConstGlobal(var);
   var->setSection(getReflectionTypeRefSectionName());