[clang][OpenMP] Fix unused var warning

This was added by 453e02ca.Use
isa instead since we don't use the result.

Fixes:
<..>SemaOpenMP.cpp:23149:13: warning: unused variable ‘TargetVarDecl’ [-Wunused-variable]
23149 |   if (auto *TargetVarDecl = dyn_cast_or_null<VarDecl>(TargetDecl))
      |             ^~~~~~~~~~~~~

Which came up when building with GCC 9.