Wrap the new ext-warn on extraneous comma on the last entry of an enumerator

Wrap the new ext-warn on extraneous comma on the last entry of an enumerator
list in a diagnostic group so it can be turned on and off. A terrifying amount
of code, including large chunks of open source code, still do this so it's
important to be able to suppress it when necessary.

Doug, is this a reasonable compromise? I'd lean toward making it a normal
extension, but I don't feel strongly as long as we can turn the warnings off.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107865 91177308-0d34-0410-b5e6-96231b3b80d8