[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
Re-commit after revert in r367773 because r367755 changed the LLVM-IR
output such that a CHECK line failed.
Patch by Lingda Li <email@example.com>
Differential Revision: https://reviews.llvm.org/D59474