[OPENMP] Allow use of declare target directive inside struct

[OPENMP] Allow use of declare target directive inside struct
declaration.

Patch allows using of the `#pragma omp declare target`| `#pragma omp end
declare target` directives inside the structures if we need to mark as
declare target only some static members.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314833 91177308-0d34-0410-b5e6-96231b3b80d8