[CUDA] Let NVPTX inherit the host's calling conventions.
Summary:
When compiling device code, we may still see host code with explicit
calling conventions. NVPTX needs to claim that it supports these CCs,
so that (a) we don't raise noisy warnings, and (b) we don't break
existing code which relies on the existence of these CCs when
specializing templates. (If a CC doesn't exist, clang ignores it, so
two template specializations which are different only insofar as one
specifies a CC are considered identical and therefore are an error if
that CC is not supported.)
Reviewers: tra
Subscribers: cfe-commits
Differential Revision: https://reviews.llvm.org/D28323
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@291136
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