[HIP][Clang][Driver] Add Driver support for `hipstdpar`

This patch adds the Driver changes needed for enabling HIP parallel algorithm offload on AMDGPU targets. What this change does can be summed up as follows:

- add two flags, one for enabling `hipstdpar` compilation, the second enabling the optional allocation interposition mode;
- the flags correspond to new LangOpt members;
- if we are compiling or linking with --hipstdpar, we enable HIP; in the compilation case C and C++ inputs are treated as HIP inputs;
- the ROCm / AMDGPU driver is augmented to look for and include an implementation detail forwarding header; we error out if the user requested `hipstdpar` but the header or its dependencies cannot be found.

Tests for the behaviour described above are also added.

Reviewed by: MaskRay, yaxunl

Differential Revision: https://reviews.llvm.org/D155775