[Attributes] Add AttributeList ctor from AttributeSet (NFC)

It was already possible to create an AttributeList from an Index
and an AttributeSet. However, this would actually end up using
the implicit constructor on AttrBuilder, thus doing an unnecessary
conversion from AttributeSet to AttrBuilder to AttributeSet.
Instead we can accept the AttributeSet directly, as that is what
we need anyway.

Author: nikic

llvm/include/llvm/IR/Attributes.h

@@ -455,6 +455,8 @@ class AttributeList {
                            ArrayRef<uint64_t> Values);
   static AttributeList get(LLVMContext &C, unsigned Index,
                            ArrayRef<StringRef> Kind);
+  static AttributeList get(LLVMContext &C, unsigned Index,
+                           AttributeSet Attrs);
   static AttributeList get(LLVMContext &C, unsigned Index,
                            const AttrBuilder &B);
 

llvm/lib/IR/Attributes.cpp

@@ -1136,15 +1136,20 @@ AttributeList AttributeList::get(LLVMContext &C, AttributeSet FnAttrs,
 }
 
 AttributeList AttributeList::get(LLVMContext &C, unsigned Index,
-                                 const AttrBuilder &B) {
-  if (!B.hasAttributes())
+                                 AttributeSet Attrs) {
+  if (!Attrs.hasAttributes())
     return {};
   Index = attrIdxToArrayIdx(Index);
   SmallVector<AttributeSet, 8> AttrSets(Index + 1);
-  AttrSets[Index] = AttributeSet::get(C, B);
+  AttrSets[Index] = Attrs;
   return getImpl(C, AttrSets);
 }
 
+AttributeList AttributeList::get(LLVMContext &C, unsigned Index,
+                                 const AttrBuilder &B) {
+  return get(C, Index, AttributeSet::get(C, B));
+}
+
 AttributeList AttributeList::get(LLVMContext &C, unsigned Index,
                                  ArrayRef<Attribute::AttrKind> Kinds) {
   SmallVector<std::pair<unsigned, Attribute>, 8> Attrs;