[OPENMP50]Codegen for nontemporal clause.

Summary:
Basic codegen for the declarations marked as nontemporal. Also, if the
base declaration in the member expression is marked as nontemporal,
lvalue for member decl access inherits nonteporal flag from the base
lvalue.

Reviewers: rjmccall, hfinkel, jdoerfert

Subscribers: guansong, arphaman, caomhin, kkwli0, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71708