IRGen: address TODO for COMDATing

Enable COMDATing in `createFunction`.  This is particularly important
for the emission of the GetEnumTagSinglePayload.  The function emission
is marked as linkonce ODR but does not get COMDAT'ed currently, breaking
emission on PE/COFF targets like Windows.

Author: compnerd

lib/IRGen/GenDecl.cpp

@@ -1594,9 +1594,6 @@ llvm::Function *irgen::createFunction(IRGenModule &IGM,
 
   llvm::Function *fn =
     llvm::Function::Create(signature.getType(), linkInfo.getLinkage(), name);
-  // TODO(compnerd) apply COMDAT to definitions
-  fn->setVisibility(linkInfo.getVisibility());
-  fn->setDLLStorageClass(linkInfo.getDLLStorage());
   fn->setCallingConv(signature.getCallingConv());
 
   if (insertBefore) {
@@ -1605,6 +1602,9 @@ llvm::Function *irgen::createFunction(IRGenModule &IGM,
     IGM.Module.getFunctionList().push_back(fn);
   }
 
+  ApplyIRLinkage({linkInfo.getLinkage(), linkInfo.getVisibility(), linkInfo.getDLLStorage()})
+      .to(fn);
+
   llvm::AttrBuilder initialAttrs;
   IGM.constructInitialFnAttributes(initialAttrs, FuncOptMode);
   // Merge initialAttrs with attrs.