vtkParseExtras, vtkParsePreprocess: use static storage for returns

GCC 10 warns that pointers to the stack are being returned. Use static
storage for this instead. This code is not thread-safe or re-entrant
anyways, so this is fine.
Status Job ID Name Coverage
  External
passed cdash-buildbot

passed ghostflow-branch-check-master

failed vtk@master-adora-linux-shared-release+cuda+gcc+mpi+optix+python2+tbb+vtkm

00:13:38

failed vtk@master-bigmac-osx-shared-debug+adios2+clang+python2+vtkm

00:01:07

failed vtk@master-duma-linux-shared-release+osmesa+ospray

00:04:27

running vtk@master-eeloo-linux-shared-release+adios2+extdeps+java+mpi+python3+qt+tbb+vtkm

9106:07:29

running vtk@master-eeloo-linux-shared-release+mpi+nogl

9106:07:30

failed vtk@master-luigi-linux-shared-release+mpi+python2+tbb

00:16:15

failed vtk@master-mun-windows-shared-release+kits+mpi+python3+qt+tbb

00:06:49

failed vtk@master-mun-windows-shared-release+mpi+python3+tbb+vtkm

00:05:38

failed vtk@master-mun-windows-static-release+mpi+qt+tbb

00:05:23

failed vtk@master-taanab-linux-shared-release+adios2+mpi+python2

00:04:19

failed vtk@master-trey-osx-shared-release+python3+qt

00:09:34