[OpenMP] Fix memory leak when translating arguments

[OpenMP] Fix memory leak when translating arguments

Parsing the argument after -Xopenmp-target allocates memory that needs
to be freed. Associate it with the final DerivedArgList after we know
which one will be used.

Differential Revision: https://reviews.llvm.org/D38257

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314328 91177308-0d34-0410-b5e6-96231b3b80d8