[HIP][LLVM][Opt] Add LLVM support for `hipstdpar`

This patch adds the LLVM changes needed for enabling HIP parallel algorithm offload on AMDGPU targets. What we do here is add two passes, one mandatory and one optional:

1. HipStdParAcceleratorCodeSelectionPass is mandatory, depends on CallGraphAnalysis, and implements the following transform:

    - Traverse the call-graph, and check for functions that are roots for accelerator execution (at the moment, these are GPU kernels exclusively, and would originate in the accelerator specific algorithm library the toolchain uses as an implementation detail);
    - Starting from a root, do a BFS to find all functions that are reachable (called directly or indirectly via a call- chain) and record them;
    - After having done the above for all roots in the Module, we have the computed the set of reachable functions, which is the union of roots and functions reachable from roots;
    - All functions that are not in the reachable set are removed; for the special cas...