PR19562: DebugInfo temporary MDNode leak: Don't include a temporary node to replace with a variable list for methods, since they're always declarations and thus never include variables

This field is used for a list of variables to ensure they are not lost
during optimization (they're only included when optimizations are
enabled).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208159 91177308-0d34-0410-b5e6-96231b3b80d8