[OpenMP 5.0] Codegen support for user-defined mappers.

This patch implements the code generation for OpenMP 5.0 declare mapper
(user-defined mapper) constructs. For each declare mapper, a mapper
function is generated. These mapper functions will be called by the
runtime and/or other mapper functions to achieve user defined mapping.

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/D59474

llvm-svn: 367773