[TableGen] Let list elements have a trailing comma

[javed.absar]

Summary

Let lists have an trailing comma to allow cleaner diffs e.g:

def : Features<[FeatureA,
                           FeatureB,
                          ]>;

I could extend it to template arguments, if there is enough consensus on it.

[javed.absar]

ping!

[hfinkel]

LGTM

lib/TableGen/TGParser.cpp

void TGParser::ParseValueList(SmallVectorImpl<Init*> &Result, Record *CurRec,
   if (!Result.back()) {
     Result.clear();
     return;
   }
 
   while (Lex.getCode() == tgtok::comma) {
     Lex.Lex();  // Eat the comma
 
+    // ignore trailing comma for lists
+    if (Lex.getCode() == tgtok::r_square)
+      return;
+
     if (ArgsRec && !EltTy) {
       ArrayRef<Init *> TArgs = ArgsRec->getTemplateArgs();
       if (ArgN >= TArgs.size()) {
         TokError("too many template arguments");
         Result.clear();
         return;
       }
       const RecordVal *RV = ArgsRec->getValue(TArgs[ArgN]);

test/TableGen/ListArgs.td

 // RUN: llvm-tblgen %s
 // XFAIL: vg_leak
 
 class B<list<int> v> {
   list<int> vals = v;
 }
 
 class BB<list<list<int>> vals> : B<vals[0]>;
 class BBB<list<list<int>> vals> : BB<vals>;
 
 def OneB : BBB<[[1,2,3]]>;
 def TwoB : BBB<[[1,2,3],[4,5,6]]>;
+
+def ThreeB: BBB<[[1,2,3],
+                 [4,5,6],
+                ]>;