Reland "[IR] Make AttributeSetNode public, avoid temporary AttributeList copies"

This re-lands r299875.

I introduced a bug in Clang code responsible for replacing K&R, no
prototype declarations with a real function definition with a prototype.
The bug was here:

       // Collect any return attributes from the call.
  -    if (oldAttrs.hasAttributes(llvm::AttributeList::ReturnIndex))
  -      newAttrs.push_back(llvm::AttributeList::get(newFn->getContext(),
  -                                                  oldAttrs.getRetAttributes()));
  +    newAttrs.push_back(oldAttrs.getRetAttributes());

Previously getRetAttributes() carried AttributeList::ReturnIndex in its
AttributeList. Now that we return the AttributeSetNode* directly, it no
longer carries that index, and we call this overload with a single node:
  AttributeList::get(LLVMContext&, ArrayRef<AttributeSetNode*>)

That aborted with an assertion on x86_32 targets. I added an explicit
triple to the test and added CHECKs to help find issues like this in the
future sooner.

llvm-svn: 299899
diff --git a/llvm/lib/Transforms/Utils/CodeExtractor.cpp b/llvm/lib/Transforms/Utils/CodeExtractor.cpp
index a7f7993..644d93b 100644
--- a/llvm/lib/Transforms/Utils/CodeExtractor.cpp
+++ b/llvm/lib/Transforms/Utils/CodeExtractor.cpp
@@ -362,8 +362,7 @@
   //  "target-features" attribute allowing it to be lowered.
   // FIXME: This should be changed to check to see if a specific
   //           attribute can not be inherited.
-  AttributeList OldFnAttrs = oldFunction->getAttributes().getFnAttributes();
-  AttrBuilder AB(OldFnAttrs, AttributeList::FunctionIndex);
+  AttrBuilder AB(oldFunction->getAttributes().getFnAttributes());
   for (const auto &Attr : AB.td_attrs())
     newFunction->addFnAttr(Attr.first, Attr.second);