[HIP][Clang][CodeGen] Simplify test for `hipstdpar`

Fixes build failures for cases where there's no additional visibility / linkage spec.

Differential Revision: https://reviews.llvm.org/D155850

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

This patch adds the CodeGen changes needed for enabling HIP parallel algorithm offload on AMDGPU targets. This change relaxes restrictions o

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

This patch adds the CodeGen changes needed for enabling HIP parallel algorithm offload on AMDGPU targets. This change relaxes restrictions on what gets emitted on the device path, when compiling in `hipstdpar` mode:

1. Unless a function is explicitly marked `__host__`, it will get emitted, whereas before only `__device__` and `__global__` functions would be emitted;
2. Unsupported builtins are ignored as opposed to being marked as an error, as the decision on their validity is deferred to the `hipstdpar` specific code selection pass;
3. We add a `hipstdpar` specific pass to the opt pipeline, independent of optimisation level:
- When compiling for the host, iff the user requested it via the `--hipstdpar-interpose-alloc` flag, we add a pass which replaces canonical allocation / deallocation functions with accelerator aware equivalents.

A test to validate that unannotated functions get correctly emitted is added as well.

Reviewed by: yaxunl, efriedma

Differential Revision: https://reviews.llvm.org/D155850

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