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

[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

Re-commit after revert in r367773 because r367755 changed the LLVM-IR
output such that a CHECK line failed.

Patch by Lingda Li <lildmh@gmail.com>

Differential Revision: https://reviews.llvm.org/D59474

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367905 91177308-0d34-0410-b5e6-96231b3b80d8