[OpenMP][IRBuilder] `omp task` support

This patch adds basic support for `omp task` to the OpenMPIRBuilder.

The outlined function after code extraction is called from a wrapper function with appropriate arguments. This wrapper function is passed to the runtime calls for task allocation.

This approach is different from the Clang approach - clang directly emits the runtime call to the outlined function. The outlining utility (OutlineInfo) simply outlines the code and generates a function call to the outlined function. After the function has been generated by the outlining utility, there is no easy way to alter the function arguments without meddling with the outlining itself. Hence the wrapper function approach is taken.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D71989