Fix Altivec vector literal parser hack for C++11.

It doesn't make any sense to accept "..." in the argument to a C-style cast,
so use a separate expression list parsing routine which rejects it. PR16874.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188330 91177308-0d34-0410-b5e6-96231b3b80d8