switch TemplateArgumentListBuilder to hold its flat argument list in a smallvector

instead of new[]'d.  This greatly reduces the number of new[]'s, and guess what, 
they were all leaked.

This adds a fixme in this hunk:

   unsigned NumPackArgs = NumFlatArgs - PackBeginIndex;
+  // FIXME: NumPackArgs shouldn't be negative here???
   if (NumPackArgs)
-    PackArgs = &FlatArgs[PackBeginIndex];
+    PackArgs = FlatArgs.data()+PackBeginIndex;

where test/SemaTemplate/variadic-class-template-2.cpp is accessing the vector
out of range and NumPackArgs is negative.  I assume variadic template args are
completely hosed.

llvm-svn: 104194