[NFC][AttributeList] Replace index_begin/end with an iterator

We expose the fact that we rely on unsigned wrapping to iterate through
all indexes. This can be confusing. Rather, keeping it as an
implementation detail through an iterator is less confusing and is less
code.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D110885

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -835,7 +835,7 @@ void ModuleBitcodeWriter::writeAttributeTable() {
   SmallVector<uint64_t, 64> Record;
   for (unsigned i = 0, e = Attrs.size(); i != e; ++i) {
     AttributeList AL = Attrs[i];
-    for (unsigned i = AL.index_begin(), e = AL.index_end(); i != e; ++i) {
+    for (unsigned i : AL.indexes()) {
       AttributeSet AS = AL.getAttributes(i);
       if (AS.hasAttributes())
         Record.push_back(VE.getAttributeGroupID({i, AS}));