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

This patch adds the Driver changes needed for enabling HIP parallel algorithm offload on AMDGPU targets. This change merely adds two macros to inform user space if we are compiling in `hipstdpar` mode and, respectively, if the optional allocation interposition mode has been requested, as well as associated minimal tests. The macros can be used by the runtime implementation of offload to drive conditional compilation, and are only defined if the HIP language has been enabled.

Reviewed by: yaxunl

Differential Revision: https://reviews.llvm.org/D155826