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/MetadataRequest.cpp

@@ -248,10 +248,7 @@ llvm::Constant *IRGenModule::getAddrOfStringForMetadataRef(
                                       nullptr,
                                       symbolName);
 
-  ApplyIRLinkage({llvm::GlobalValue::LinkOnceODRLinkage,
-    llvm::GlobalValue::HiddenVisibility,
-    llvm::GlobalValue::DefaultStorageClass})
-  .to(var);
+  ApplyIRLinkage(IRLinkage::InternalLinkOnceODR).to(var);
   if (alignment)
     var->setAlignment(alignment);
   setTrueConstGlobal(var);