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

This patch adds the Sema changes needed for enabling HIP parallel algorithm offload on AMDGPU targets. This change impacts the CUDA / HIP language specific checks, and only manifests if compiling in `hipstdpar` mode. In this case, we essentially do three things:

1. Allow device side callers to call host side callees - since the user visible HLL would be standard C++, with no annotations / restriction mechanisms, we cannot unambiguously establish that such a call is an error, so we conservatively allow all such calls, deferring actual cleanup to a subsequent pass over IR;
2. Allow host formed lambdas to capture by reference;
3. Allow device functions to use host global variables.

Reviewed by: yaxunl

Differential Revision: https://reviews.llvm.org/D155833