[OpenMP 5.0] Fix user-defined mapper lookup in sema

This patches fixes the case when a user-defined mapper is attached to
the elements of an array, and to report error when a mapper is used for
types other than struct, class, and union.

Patch by Lingda Li <lildmh@gmail.com>

Differential Revision: https://reviews.llvm.org/D67978

llvm-svn: 373023