Improve handling of VAArg nodes, fixes PR19175.

Reduced version of test from bug report included.

Thanks Sharan Santhanam for finding and reporting this!

git-svn-id: https://llvm.org/svn/llvm-project/poolalloc/trunk@208097 91177308-0d34-0410-b5e6-96231b3b80d8
Disable -Werror to avoid build errors on user machines with untested compilers.

It was already an aggressive default, but no longer makes
sense due to lack of persistent project maintenance.

(Causing only pain for users uninterested in debugging warnings)

git-svn-id: https://llvm.org/svn/llvm-project/poolalloc/trunk@208100 91177308-0d34-0410-b5e6-96231b3b80d8