[OPENMP]Fix PR46012: declare target pointer cannot be accessed in target region.

Summary:
Need to avoid an optimization for base pointer mapping for target data
directives.

Reviewers: jdoerfert, ye-luo

Subscribers: yaxunl, guansong, cfe-commits, sstefan1, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84182