[OPENMP] Codegen for `omp declare target` construct. Added initial codegen for device side of declarations inside `omp declare target` construct + codegen for implicit `declare target` functions, which are used in the target regions. git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@327636 91177308-0d34-0410-b5e6-96231b3b80d8