[OpenMP 5.0] libomptarget interface for declare mapper functions.

This patch implements the libomptarget runtime interface for OpenMP 5.0
declare mapper functions. The declare mapper functions generated by
Clang will call them to complete the mapping of members.
kmpc_mapper_num_components gets the current number of components for a
user-defined mapper; kmpc_push_mapper_component pushes back one
component for a user-defined mapper.

The design slides can be found at
https://github.com/lingda-li/public-sharing/blob/master/mapper_runtime_design.pptx

Patch by Lingda Li <lildmh@gmail.com>

Differential Revision: https://reviews.llvm.org/D60972

llvm-svn: 367772